Colouration in Animals and Plants

by Alfred Tylor

This appears to be the case, for Engelmann has shown[9] that many of the lowest forms of life, which are almost mere specks of protoplasm, are influenced by light, some seeking and others shunning it. He found, too, that in the case of Euglena viridis it would seek the light only if it "were allowed to fall upon the anterior part of the body. Here there is a pigment spot; but careful experiment showed that this was not the point most sensitive to light, a colourless and transparent area of protoplasm lying in front of it being found to be so." Commenting upon this Romanes observes, "it is doubtful whether this pigment spot is or is not to be regarded as an exceedingly primitive organ of special sense." Haeckel has also made observations upon those lowest forms of life, which, being simply protoplasm without the slightest trace of organization, not even possessing a nucleus, form his division Protista, occupying the no-man's-land between the animal and vegetable kingdoms. He finds that "already among the microscopic Protista there are some that love light, and some that love darkness rather than light. Many seem also to have smell and taste, for they select their food with great care.... Here, also, we are met by the weighty fact that sense-function is possible without sense organs, without nerves. In place of these, sensitiveness is resident in that wondrous, structureless, albuminous substance, which, under the name of protoplasm, or organic formative material, is known as the general and essential basis of all the phenomena of life."[10]

Now, whether Romanes be correct in doubting whether the pigment-spot in Euglena is a sense organ or not, matters little to our present enquiry, but it certainly does seem that the spot, with its accompanying clear space, looks like such an organ. And when we are further told that after careful experiment it is found that Euglena viridis prefers blue to all the colours of the spectrum, the fundamental fact seems to be established that even as low down as this the different parts of the spectrum affect differently the body of creatures very nearly at the bottom of the animal scale. This implies a certain selection of colour, and, equally, an abstention from other colours.

It is not part of our scheme, however, to follow out in detail the development of the organs of special sense, and the reader must be referred to the various works of Mr. Romanes, who has worked long and successfully at this and kindred problems. Suffice it to say that in this and other cases he has been led to adopt the theory of inherited memory, though not, as we believe, in the fulness with which it must ultimately be acquired.

This, however, seems certain, that the development, not only of the sense organs, but of organs in general—that is, the setting aside of certain portions for the performance of special duties, and the modifications of those parts in relation to their special duties, is closely related to the activity of the organism. Thus, we find in those animals, like some of the Cœlenterata, which pass some portion of their existence as free-swimming beings, and the remainder in a stationary or sessile condition, that the former state is the most highly organized. This is shown to a very remarkable degree in the Sea Squirts (Ascidians), a class of animals that are generally grouped with the lower Mollusca, but which Prof. Ray Lankester puts at the base of the Vertebrata.

These animals are either solitary or social, fixed or free; but even when free, have little or no power of locomotion, simply floating in the sea. Their embryos are, however, free-swimming, and some of the most interesting beings in nature. Some are marvellously like young tadpoles, and possess some of the distinctive peculiarities of the Vertebrata. Thus, the body is divided into a head and body, or tail, as in tadpoles. The head contains a large nerve centre, corresponding with the brain, which is produced backwards into a chord, corresponding to the spinal chord. In the head, sense organs are clearly distinguishable; there is a well-marked eye, an equally clear ear, and a less clearly marked olfactory organ. Besides this, the spinal-cord is supported below by a rod-like structure, called the notochord. In the vertebrate embryo this structure always precedes the development of the true vertebral column, and in the lowest forms is persistent through life.

We have thus, in the ascidian larva, a form which, if permanent, would most certainly entitle it to a place in the vertebrate sub-kingdom. It is now an active free-swimming creature, but as maturity approaches it becomes fixed, or floating, and all this pre-figurement of a high destiny is annulled. The tail, with its nervous cord and notochord atrophies, and in the fixed forms, not only do the sense organs pass away, but the entire nervous system is reduced to a single ganglion, and the creature becomes little more than an animated stomach. It is, as Ray Lankester has pointed out, a case of degeneration. In the floating forms, which still possess a certain power of locomotion, this process is not carried to such extremes, and the eye is left.

Now, cases of this kind are important as illustrating the direct connection between an active life and advancement; and they also add indirectly to the view Wallace takes of colouration, namely, that the most brilliant colour is generally applied to the most highly modified parts, and is brightest in the seasons of greatest activity.

But they have a higher meaning also, for they may point us to the prime cause of the divergence of the animal and vegetable kingdoms. In thinking over this matter, one of us ventured to suggest that probably the reason why animals dominate the world, and not plants, is, that plants are, as a rule, stationary, and animals lead an active existence. We can look back to the period prior to the divergence of living protoplasm into the two kingdoms. Two courses only were open to it, either to stay at home, and take what came in its way, or to travel, and seek what was required. The stay-at-homes became plants, and the gad-abouts animals. In a letter it was thus put; "It is a truly strange fact that a free-swimming, sense-organ-bearing animal should degenerate into a fixed feeding and breeding machine. It seems to me that the power of locomotion is a sine qua non for active development of type, as it necessarily sharpens the wits by bringing fresh experiences and unlooked-for adventures to the creature. I almost think, and this, I believe may be a great fundamental fact, that the only reason why animals rule the world instead of plants is that plants elected to stay at home, and animals did not. They had equal chances. Both start as active elements; the one camps down, and the other looks about him."

Talking over this question with Mr. Butler, he astonished the writer by quoting from his work, "Alps and Sanctuaries" (p. 196), the following passage:—

"The question of whether it is better to abide quiet, and take advantage of opportunities that come, or to go farther afield in search of them, is one of the oldest which living beings have to deal with. It was on this that the first great schism or heresy arose in what was heretofore the catholic faith of protoplasm. The schism still lasts, and has resulted in two great sects—animals and plants. The opinion that it is better to go in search of prey is formulated in animals; the other—that it is better, on the whole, to stay at home, and profit by what comes—in plants. Some intermediate forms still record to us the long struggle during which the schism was not yet complete.