To my mind, all this is but the normal result of the divergence of character, or the survival of the unlike. A new type finds places of least conflict, it spreads rapidly and widely, and thereby varies immensely. It is a generalized type, and therefore adapts itself at once to many and changing conditions. A virile plant is introduced into a country in which the same or similar plants are unknown, and immediately it finds its opportunity and becomes a weed, by which we mean that it spreads and thrives everywhere. Darwin and Gray long ago elucidated this fact. The trilobites, spirifers, conifers, ginkgos, were weed types of their time, the same as the composites are to-day. They were stronger than their contemporaries, the same as our own weeds are stronger than the cultivated plants with which they grow. After a time the new types outran their opportunity, the remorseless struggle for existence tightened in upon them, the intermediate unlikenesses had been blotted out, and finally only one or two types remained, struggling on through the ages, but doomed to perish with the continuing changes of the earth. They became specialized and inelastic; and the highly specialized is necessarily doomed to extinction. Such remnants of a vanquished host remain to us in our single liriodendron, the single ginkgo, and sassafras, and the depleted ranks of the conifers. My attention was first called to this line of thought by contemplating upon the fact that cultivated plants differ widely in variability, and I was struck by the fact that many of our most inextricably variable groups-as the cucurbits, maize, citrus, and the great tribes of composites are still unknown in a fossil state, presumably because of their recent origin. Many other variable genera, to be sure, are well represented in fossil species, as roses (although these are as late as the Eocene), pyrus, prunus, and musa; but absolute age is not so significant as the comparative age of the type, for types which originated very far back may be yet in the comparative youth of their development. The summary conclusions of a discussion of this subject were presented to the American Association for the Advancement of Science two years ago. A modification of these points, as I now understand them, would run something as follows:

1. There is a wide difference in variability in cultivated plants. Some species vary enormously, and others very little.

2. This variability is not correlated with age of cultivation, degree of cultivation, or geographical distribution.

3. Variability of cultivated plants must be largely influenced and directed, therefore, by some antecedent causes.

4. The chief antecedent factor in directing this variability is probably the age of the type. New types in geologic time, are polymorphous; old types are monomorphous and are tending toward extinction. The most flexible types of cultivated plants are such as have probably not yet passed their zenith, as the cucurbits, composites, begonias, and the

I Proc. A. A. A. S. 1894, 255; Botanical Gazette, XIX, 381.

like. The varieties of cereals, which are old types, are so much alike that expert knowledge is needed to distinguish them.

5. New types are more variable and flexible because less perfectly molded into and adjusted to the circumstances of life than the old types are. They have not yet reached the limits of their dissemination and variation. They are generalized forms.

The reader will please observe that I have here regarded the origin and survival of the unlike in the plant creation in the sense of a plastic material which is acted upon by every external stimulus, and which must necessarily vary from the very force of its acquired power of growth, and the unlikenesses are preserved because they are unlike. I have no sympathy with the too prevalent idea that all the attributes of plants are direct adaptations, or that they are developed as mere protections from environment and associates. There is a type of popular writings which attempts to evolve many of the forms of plants as a mere protection from assumed enemies. Perhaps the plant features which have been most abused in this manner are the spines, prickles, and the like, and the presence of acrid or poisonous qualities. As a sample of this type of writing, I will make an extract from Massee's Plant World:

"Amongst the most prominent and general modes of protection of vegetative parts against the attacks of living enemies may be mentioned prickles, as in roses and brambles, which may either be straight, and thus prevent the nibblings of animals, or, in more advanced species, curved, thus enabling the weak stem to climb and carry its leaves out of harm's way. Spines that are sharp-pointed abortive branches, serving the same purpose as prickles, as in the common sloe or blackthorn (Prunus spinosa). Rigid hairs on leaves and stem, as in the borage (Borago officinalis) and comfrey (Symphytum officinale). Stinging hairs, as in the common nettles (Urtica dioica and U. urens). In these cases the stinging hairs are mixed on the leaves and stem with ordinary rigid hairs, of which they are higher developments, distinguished by the lower or basal swollen portion of the hair containing an irritating liquid that is ejected when the tip of the hair is broken off. Bitter taste, often accompanied by a strong scent, as in wormwood (Artemisia vulgaris), chamomile (Anthemis nobilis), and the leaves and fruit of the walnut (Juglans regia). Poisonous alkaloids, as in the species of Strychnos, which contain two very poisonous alkaloids, strychnine and brucine, in the root and the seeds; decoctions of species of Strychnos are used by the Javanese and the natives of South America to poison their arrows. Some of the species, as Strychnos nua vomica, are valuable medicines, depending on the strychnine they contain, which acts as a powerful excitant of the spinal cord and nerves; thus the most effective protective arrangements evolved by plants can be turned to account, and consequently lead to the destruction of the individuals they were designed to protect. Our common arum (Arum maculatum), popularly known as 'Lords and Ladies,' has an intensely acrid substance present in the leaves, which effectually protects it from the attacks of mammals and caterpillars, but not from the attacks of parasitic fungi, which appear to be indifferent to all protective contrivances exhibited by plants, nearly every plant supporting one or more of these minute pests, the effects of which will be realized by mentioning the potato disease, 'rust' and 'smut' in the various cereals, and the hop disease, all due to parasitic fungi.”

Now, this is merely a gratuitous and ad captandum species of argument, one which is designed to please the fancy and to satisfy those superficial spirits who are still determined to read the element of design into organic nature. It does not account for the facts. These particu lar attributes of plants are specialized features, and it is always unsafe to generalize upon specializations. Each and every one of such specialized features must be investigated for itself. Probably the greater number of spinous processes will be found to be the residua following the contraction of the plant body; others are no doubt mere correlatives of the evolution of other attributes; and some may be the eruptions of the growth force; and the acrid and poisonous properties are quite as likely to be wholly secondary and useless features. The attempt to find a definite immediate use and office for every attribute in the creation is superficial and pernicious. There are many attributes of organisms which are not only useless, but positively dangerous to the possessor, and they can be understood only as one studies them in connection with the long and eventful history of the line of ascent.

The thought which I want to leave with you, therefore, is that unlikenesses are the greatest facts in the organic creation. These unlikenesses in plants are (1) the expressions of the ever-changing environmental conditions in which plants grow, and of the incidental stimuli to which they are exposed; (2) the result of the force of mere growth; (3) the outcome of sexual mixing. They survive because they are unlike, and thereby enter fields of least competition. The possibility of the entire tragic evolution lay in the plasticity of the original life-plasma. The plastic creation has grown into its own needs day by day and age by age, and it is now just what it has been obliged to be. It could have been nothing else.

THE LAW WHICH UNDERLIES PROTECTIVE COLORATION.1

By ABBOTT H. THAYER.

This article is intended to set forth a beautiful law of nature which, so far as I can discover, has never been pointed out in print. It is the law of gradation in the coloring of animals, and is responsible for most of the phenomena of protective coloration except those properly called mimicry.

Naturalists have long recognized the fact that the coloring of many animals makes them difficult to distinguish, and have called the whole phenomenon protective coloration, little guessing how wonderful a fact lay hidden under the name.

Mimicry makes an animal appear to be some other thing, whereas this newly discovered law makes him cease to appear to exist at all. The following are some examples of true mimicry. The screech owl, when startled, makes himself tall and slim, and with eyes shut to a narrow line simulates a dead stub of the tree on which he sits. Certain herons stretch their necks straight upward, and with head and green beak pointed at the zenith pass themselves off for blades of sedge grass. Certain harmless snakes spread their heads out flat, in imitation of their poisonous cousins, and rattle with their tails in the leaves. Many butterflies have stone or bark colored under sides to their wings, which make them look like a bit of bark or lichen when they sit still on a stone or tree trunk with wings shut over their backs.

The newly discovered law may be stated thus: Animals are painted by nature darkest on those parts which tend to be most lighted by the sky's light, and vice versa.

The accompanying diagram illustrates this statement. Animals are colored by nature as in A, the sky lights them as in B, and the two effects cancel each other as in C. The result is that their gradation of light and shade, by which opaque solid objects manifest themselves to the eye, is effaced at every point, the cancellation being as complete at one point as another, as in C of the diagram, and the spectator seems to see right through the space really occupied by an opaque animal. Printed in The Auk, Vol. XIII, April and October, 1896.