Parallel Paths: A Study in Biology, Ethics, and Art

by T. W. Rolleston

These results appear to conflict most seriously with the theory of the transmissibility of acquired modifications.

Fig. 3.

Kallima paralecta, as it appears at rest, with wings closed.
From Weismann’s The Evolution Theory.
K, the head; B, the limbs.

Every one is familiar with the fact that species of animals which are preyed on by others, or which require to be inconspicuous for the purpose of preying, are very apt to take the colour of their habitual surroundings. Individuals of the same species will even differ according to their special habitat. Perhaps the most marvellous instances of this kind of adaptation are to be found in certain tropical butterflies, such as the Indian butterfly, Kallima paralecta, here illustrated. We have here, painted on the butterfly’s wing, the picture of a leaf belonging to a shrub which it frequents—a picture, when seen under natural conditions, capable of baffling all but the closest inspection. The different parts—the midrib, the lateral veinings, the little blotches and spots which represent patches of mould or drops of water, even the outer contour of the wing itself—all form an harmonious whole composed of related parts which have separately no meaning or use. They certainly did not all appear in full development at the same time. Nor could any one of them, if it appeared first, have exercised the smallest influence on the appearance of the others, as the antlers of the elk were supposed to have influenced the development of the ligamentum nuchæ. The early stages must have been anticipatory of the later ones, but exercise could have had nothing to do with the result from first to last. The butterfly never practised looking like a leaf. Nor can any large chemical and elemental influences have been at work. If nature is capable of producing such effects as this without the agency of Lamarck’s principle, are there not excellent grounds for seeking for some other agency which will cover all the phenomena alike?

Finally let us take the case of the slave-owning ‘Amazon’ ants, Polyergus rufescens. Here we have a case which at the first blush looks like a perfect picture of an evolutionary process conducted on the principles of Lamarck’s theory. These ants, it may be supposed, were originally of the ordinary type of that industrious and respectable insect, but they were led by the weakness of some of their neighbours of another species to make occasional attacks on them for the purpose of carrying off their immature brood, the pupæ, as food. Some of these pupæ, near maturity at the period of their capture, would come out while stored-up in the nest of the conquerors, and when they did so would immediately set about doing the household work of the hive as if they were at home. Polyergus rufescens ultimately became aware that a life of aristocratic leisure awaited him if he only captured enough pupæ of another species of ant to do his work. He accordingly confined himself entirely to piratical expeditions of this nature, and in the course of time underwent a moral and physical transformation of a most remarkable kind. The ordinary ant instincts have disappeared in this variety. They do not make their nests, they do not gather stores, they do not mind their young, they do not even feed themselves—an Amazon ant will perish of starvation in the presence of food if there is not a slave ant to put it into his mouth. But they fight ferociously in their slave-raids, and the form of their mandible has changed to suit their mode of life. It has become a pair of sabre-like nippers, excellent for slaying a foe, but ill-adapted for carrying objects and other industrial occupations. Corresponding changes have taken place in the head and in the chitinous and muscular structure.

We have before us, then, what would seem to an uninformed observer, a striking picture of the acquirement of a certain bodily form and a certain set of instincts by use, and the total loss of other traits by disuse, and of the fixing of these characters in a species by heredity. Yet the picture is altogether an illusion. However we are to explain the facts—of which more anon—we cannot do so by Lamarckism, for the simple reason that the peculiar instincts and bodily structure of the Amazon ants are confined to the so-called ‘worker,’ or in this case ‘soldier,’ caste, which are sexless, and incapable of reproducing their kind. If these were the individuals which originally started the slave system among the species, they could not possibly have transmitted the modifications, moral and physical, which they acquired. The queen-ants, which normally are the only fertile ants, transmit them, but do not possess them, and neither do the drones.

The case of these mysterious communities of insects, composed largely of neuters which do the work of the community but do not reproduce their kind, was one of the difficulties in the way of Darwin’s theory of evolution which, he said, staggered him every time he reflected on it.[64] It is not surprising, therefore, that this difficulty came to be the battlefield, or a main position thereof, in a most interesting and illuminating controversy on Natural Selection versus Lamarckism, waged between Mr. Herbert Spencer and Dr. Weismann in the years 1893-4.[65] Spencer considered the inheritance of acquired characteristics a factor in evolution of the very first importance; and so, indeed, from his point of view it is. “Either,” he declared, “there has been inheritance of acquired characteristics, or there has been no evolution.” Met by the case, among others, of the slave-making ants, his explanation is substantially as follows: It was not the workers (soldiers) which originally acquired military traits, but the queens, the fully developed females, which lost them. There was once, as every one admits, a time when all ants, bees, etc. were sexually mature. There were only males and females. At this stage, possibly, the Amazon ants were already predatory. It was then that they may have acquired the military habits and structure, which they were then able to perpetuate by inheritance.

How, then, did the queens lose these traits?” From the queens,” replies Spencer, “they have slowly disappeared by inheritance of the effects of disuse.” The obvious and unanswerable rejoinder made by Weismann and his followers was that Spencer had only shifted the difficulty to another ground—from the workers to the queens. If the queens (and drones) lost the military characteristics by disuse, how do they come to transmit them unimpaired to the workers? It is the very essence of Lamarckism that whatever modifications are produced by use or by disuse shall be transmissible by inheritance.

In this controversy, however, there was another string to the Lamarckian bow. Worker-ants, bees, etc. are imperfectly developed females. They have four or five egg-tubes where the queen has two hundred, but they cannot be fertilized by the drones. It occasionally happens, however, that these neuter insects do lay a few eggs. These unfertilized eggs always develop into drones. One of these drones might, it was suggested, now and then fertilize a genuine queen, and thus hand on the traits of the worker from which it sprang. But apart from the fact that an occasional occurrence of this sort would hardly suffice to maintain the worker-characteristics unimpaired throughout the ages, there is the decisive answer, as Weismann points out, that we know at least one species of ant in which the evolution of a neuter caste is absolutely complete, for the workers of Tetramorium caespitum possess no egg-tubes at all. Yet the transmission of characteristics from queens and drones who never exercise them to workers who cannot pass them on, goes forward in this species of any ant just as in any other.