Before describing and discussing the special facts to which they apply their theory, it is necessary to examine its biological foundation.

CHAPTER I

THE PATH OF DEGENERATION IN BIOLOGY

WE have now to consider if the degeneration of organs retraces the steps taken in their progressive evolution. According to Hughlings Jackson and Ribot, in the cases mentioned in the preceding chapter, degeneration proceeds by successive atrophies occurring in the order opposite to that of ontological formation. Is the same order to be found when we compare the degeneration of organs or of individuals with their phylogenetic development? To answer this question, we must employ both morphology and embryology. Using the morphological method, we shall study the reduction of a homologous organ in several species descended from the same type, and compare the different stages of reduction with the different stages of phylogenetic development of the organ.1

1 In this investigation it will be necessary to compare absolutely identical organs-for instance, not to compare the pineal eye with the paired eyes. It will be necessary also to choose animals of common parentage—to avoid, for instance, comparing a Vertebrate eye with a Crustacean eye.

Using the method of embryology and the principle, so far as it can be followed, that ontogeny recapitulates phylogeny, we shall investigate the mode of origin of some reduced structures. We shall thus learn if the organs in degenerating resume any of their ancestral stages.

Reduced organs may appear in two different forms. They may be atrophied after having reached a more or less complete development, and in this case we shall have to compare the course of the atrophy with the course of the development. They may be rudimentary, that is to say, their development may have been arrested at a given point, the adult state never being attained. In this case, so far as ontogeny repeats phylogeny, the arrest of these organs at different stages in different species should furnish a series with greater or smaller lacunæ, but a series which will be the reversed series of the original stages in phylogeny.

SECTION I.

The path of degeneration in animals.

The law of re

1. Morphology and Embryology. capitulation. It has been so often repeated that the individual development of an organ is a résumé more or less compressed of its historical evolution, that people are apt to attempt too exact an application

Such an

as

of this principle to every individual case. exact application is, however, impossible. Every living organism is plastic, and in its development presents individual variations which serve material for the operation of natural selection. In consequence, the recapitulation cannot be more than a repetition more or less vague of the essential phases of phylogeny.1

Moreover, there is nothing inevitable in the law of recapitulation, for most plants develop directly.

With these limitations, however, we may state that among animals, the ontogeny usually repeats in a modified fashion the main ancestral stages. This is certainly the case when we compare the development of the brain of man with the probable ancestral stages as displayed in the series of vertebrates.

1 Lang, Anatomie compareé. As throughout the whole course of time, adaptation, that is to say, the preservation of what is most useful in the struggle for existence, is a force modifying heredity, it is plain that a species instead of resting stable must change. According to its circumstances, moreover, the successive stages in the ontogeny of a creature are under the influence of conditions different from those that affected the corresponding ancestral stages. We shall call the process of embryology palingenetic so far as it is based upon inherited legacies, and cœnogenetic so far as it is modified by adaptation.

Baldwin in his Treatise on Mental development in the child and in the race (London, 1895), also shows that the development of the individual is not an exact repetition of ancestral stages. The development of the child exhibits "short cuts” and phases of direct development due to adaptation and destroying the exactness of the parallel with phylogeny.

Within such limits, the law of recapitulation may be applied, and the embryonic history of an individual may be considered roundly as a repetition of the essential phases of its ancestral history. We have now to consider how far a reduction by atrophy or by arrest represents a retracing of steps in evolution (fig. 61).

From this point of view, we may study the

degeneration of the pineal eye in the slow-worm, and in a series of lizards.

The pineal or median eye in the slow worm and

FIG. 61.-A, Brain of a human embryo of seven weeks; h, cerebral hemispheres; ci, intermediate brain or thalam encephalon; cm, mid-brain; cp, hind-brain. B, Brain of a human embryo about the beginning of the third month; h, cerebral hemispheres; tq, region of the corpora quadrigemina; cm, mid-brain. C, Brain of a human embryo at the middle of the third month; h, cerebral hemispheres; tq, corpora quadrigemina; ce, cerebellum. D, Human brain of the fifth embryonic month; h, cerebral hemispheres; olf, olfactory lobes; S, fissure of Sylvius; ce, cerebellum. (After Mihalkovics, Entwickelungsgeschichte des Gehirns. Leipzig, 1877.)

the lizard passes through the following stages in its individual development.1

(1) Formation of a hollow outgrowth from the roof of the third ventricle of the brain (fig. 62, D).

1 P. Francotte, Recherches sur le développement de l'épiphyse. (Thèse presentée à la Faculté de médecine de Bruxelles.) Arch. de Biologie, 1888.