Walter Holbrook Gaskell

“ The evolution of animal life on this earth can clearly, on the whole, be described as a process of upward progress culminating in the highest form—man; but it must always be remembered that whole groups of animals such as the Tunicata have been able to survive owing to a reverse process of degeneration.
If there is one organ more than another which increases in complexity as evolution proceeds, which is the most essential organ for upward progress, surely it is the central nervous system, especially that portion of it called the brain. ”

“ Since Darwin wrote the "Origin of Species," and laid down that the law of the 'survival of the fittest' is the factor upon which evolution depends, it has gradually dawned upon the scientific mind that 'the fittest' may be produced in two diametrically opposite ways: either by progress upwards to a superior form, or by degeneration to a lower type of animal. ”

“ Perhaps the most important of all the clues likely to help in the solution of the origin of vertebrates is that afforded by Geology, for although the geological record is admittedly so imperfect that we can never hope by its means alone to link together the animals at present in existence, yet it does undoubtedly point to a sequence in the evolution of animal forms, and gives valuable information as to the nature of such sequence. In different groups of animals there are times when the group can be spoken of as having attained its most flourishing period. ”

“ Nature's mistake was rectified and further evolution secured, not by degeneration in the brain-region, for that means degradation not upward progress, but by the formation of a new food-channel, in consequence of which the brain was free to develop to its fullest extent. Thus the great and mighty kingdom of the Vertebrata was evolved with its culminating organism—man—whose massive brain with all its possibilities could never have been evolved if he had still been compelled to pass the whole of his food through the narrow œsophageal tube, still existent in him as the infundibular tube. ”

“ To those who believe in Evolution and the Darwinian theory of the survival of the fittest, it is simply inconceivable that a soft-bodied animal living in the mud, blind, with a rudimentary brain and rudimentary olfactory organs, such as is postulated when we think of Balanoglossus and Amphioxus, should hold its own and come victorious out of the struggle for existence at a time when the sea was peopled with powerful predaceous scorpion- and crab-like armour-plated animals possessing a well-developed brain, good eyes and olfactory organs, and powerful means of locomotion. ”

“ What are the lines of investigation most likely to meet with success? Is it possible to lay down any laws of evolution? It is instructive to consider the nature of the investigations which have led to the two theories just mentioned, for the fundamental starting-point is remarkably different in the two cases. The one theory is based upon the study of the vertebrate itself, and especially of its central nervous system, and its supporters and upholders have been and are essentially anatomists, whose chief study is that of vertebrate and human anatomy. ”

“ The principle of degeneration as a factor in the formation of groups of animals, which are thereby enabled to survive, is nowadays universally admitted. The most striking example is to be found in the widely distributed group of Tunicata, which live, in numbers of instances, a sedentary life upon the rocks, have the appearance of very low forms of animal life, propagate by budding, have lost all the characteristics of higher forms, and yet are considered to be derived from an original vertebrate stock. ”

“ The charge of parasitism is brought against the lamprey because it is said to suck on to fishes and so obtain nutriment. It is, however, undoubtedly a free-swimming fish; and when we see it coming up the rivers in thousands to reach the spawning-beds, and sucking on to the stones on the way in order to anchor itself against the current, or holding on tightly during the actual process of spawning, it does not seem justifiable to base a charge of degeneration upon a parasitic habit, when such so-called habit simply consists in holding on to its prey until its desires are satisfied. ”

“ Not only are the tissues formed during the stages which are passed through in a free-living larval form, serviceable tissues comparable to those of adult life, but also these stages proceed at so much slower a rate than do those in the embryo in utero or in the egg, as to make the larval form much more suitable than the embryo for the investigation of ancestral problems. It is true enough that the free life of the larva may bring about special adaptations which are not of an ancestral character, as may also occur during the life of the adult ”

“ It is immediately apparent that just as the invertebrate œsophagus leads into the large cephalic stomach, so the infundibular tube leads into the large cavity of the brain known as the third ventricle, which, together with the other ventricles, forms in the embryo a large anterior dilated part of the neural tube. In the arthropod this cephalic stomach leads into the straight narrow intestine; in the vertebrate the fourth ventricle leads into the straight narrow canal of the spinal cord. ”

“ Of all the discoveries made in recent years, the discovery that the pineal gland of the vertebrate brain was originally a pair of median eyes is by far the most important clue to the ancestry of the vertebrate, for not only do they correspond exactly in position with the median eyes of the invertebrates, but, being already degenerate and functionless in the lowest vertebrate, they must have been functional in a pre-vertebrate stage, thus giving the most direct clue possible to the nature of the pre-vertebrate stage. ”

“ On the vertebrate side the lowest non-degenerate vertebrate is the lamprey, or Petromyzon, which spends a large portion of its existence in a larval stage, known as the Ammocœtes stage of the lamprey, because it was formerly considered to be a separate species and received the name of Ammocœtes. The larval stages of any animal are most valuable for the study of ancestral problems, so that it is most fortunate for the solution of the ancestry of vertebrates that Limulus on the one side and Ammocœtes on the other are available for thorough investigation and comparison. ”

“ At the present time the dominant race is man, and the struggle for existence is essentially between the members of that race, and not between them and any inferior race.
The effect of such conditions is, as Darwin has pointed out, to cause great variation in that group; in consequence of that variation and that dominance the evolution of the next higher group is brought about from some member of the dominant group. Thus the present age is the outcome of the Tertiary period, a time when giant mammals roamed the earth and left as their successors the mammals of the present day ”

“ Since the time of Darwin the accumulation of facts in its support has been so overwhelming that all zoologists look upon this question as settled, and desire now to find out the manner in which such evolution has taken place. Here two problems offer themselves for investigation, which can be and are treated separately—the one dealing with the question of those laws of heredity and variation which have brought about in the past and are still causing in the present the evolution of living beings, i.e. the causes of evolution ”

“ Further, just as in the embryo of an arthropod it is possible, with more or less accuracy, to see the number of neuromeres or original ganglia which have fused to form the supra- and infra-œsophageal portions of its brain, so also in the embryo of a vertebrate we are able at an early stage to gain an indication, more or less accurate, of the number of neuromeres which have built up the vertebrate brain. ”

“ Here, again, the same law appears; from the predominant race at the time, the next higher race arose, and arose by a most striking modification, which was the consequence of altering the medium in which it lived. By coming out of the water and living on the land, or, rather, being able to live partly on land and partly in the water, by the acquisition of air-breathing respiratory organs or lungs in addition to, and instead of, water-breathing organs or gills, the amphibian not only arose from the fish, but made an entirely new departure in the sequence of progressive forms. ”

“ For a digestive system all that is required is an arrangement for the digestion and absorption of food, a mechanism which can arise easily if some of the cells of the skin possess digestive power. Now Miss Alcock has shown that some of the surface-cells of crustaceans secrete a fluid which possesses digestive powers, and she has also shown that certain of the cells in the skin of Ammocœtes possess digestive power. ”

“ For all these reasons the lowest form of fish to be investigated, in the hopes of finding out the nature of the earliest formed fish, is not Amphioxus, but Ammocœtes, the larval form of the lamprey—a form which, as I hope to satisfy my reader after perusal of subsequent pages, more nearly resembles the ancient Cephalaspidian fishes than any other living vertebrate. ”

“ In any classification of the animal kingdom, it is clear that large groups of animals exist which have so many common characteristics as to necessitate their being placed in one larger group or kingdom; thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Cœlenterata, Mollusca, etc. ”

“ The second primary constriction separating the mid-brain from the hind-brain is still more interesting, for it is coincident with the position of the trochlear or fourth cranial nerve. In all vertebrates without exception this nerve takes an extraordinary course; all other nerves, whether cranial or spinal, pass ventralwards to reach their destination. This nerve passes dorsalwards, crosses its fellow mid-dorsally in the valve of Vieussens, where the roof of the brain is thin, and then passes out to supply the superior oblique muscle of the eye of the opposite side. ”

“ The supra-infundibular portion of the brain in vertebrates corresponds clearly to the supra-œsophageal portion of the invertebrate brain in so far that in both cases here is the seat of the will. Voluntary action is as impossible to the arthropod deprived of its supra-œsophageal ganglia as to the vertebrate deprived of its cerebrum. ”

“ Much more likely is it that a fish-like form should have been developed out of a smooth, wriggling, worm-like animal, and it is therefore to the annelids that the upholders of the theory of the reversal of surfaces look for the ancestor of the vertebrate. ”