SO PHYSIOLOGY CHAP. 



according to which the origin of species depends upon the intimate 

 constitution of the germinal matter (or idioplasm}, inasmuch as 

 this possesses an inherent tendency to perfect itself and to progress, 

 developing by a slow and continuous evolution new and more 

 complex forms, which are independent to a certain degree either 

 of the variations of the environment or of the struggle for 

 existence. 



It is undeniable that all the branches of the zoological trunk 

 exhibit a progression from the lower forms to the higher, and 

 always in a sufficiently cognate form, although the animals may 

 be subjected to very different external conditions of existence and 

 development. We see, for instance, that the eye, which in the 

 rudimentary species of animals is represented by a simple spot of 

 pigment, is provided in worms, in arthropods, in molluscs, in 

 vertebrates, with accessory apparatus, such as the lens, the vitreous 

 body, iris, choroid, etc. This tendency towards perfection, whether 

 of single organs and apparatus, or of the individual as a whole, 

 which is revealed everywhere in the organic world, must, according 

 to Nageli (since it is comparatively independent of extrinsic vital 

 conditions), find its explanation in the very being of the living- 

 substance. 



Unlike Darwinism and Larnarckisin, which accord a pre- 

 dominating importance to external causes in phylogenic evolution, 

 Niigelism assigns the maximal importance to internal causes. 

 Nageli's phylogenesis harmonises perfectly with his ontogenesis. 

 The internal causes of the transformation are perfectly analogous 

 to those by which the germ, or fertilised ovum, develops into the 

 perfect individual, and the mutilated individual is capable of 

 regenerating a missing member (e.g. a pollarded tree can recover 

 all its branches, a lizard can reproduce its lost tail, a decapitated 

 snail can reproduce its head). It is certainly within the intimate 

 physico - chemical structure of the idioplasm of the egg, or 

 mutilated individual, and not in the environment, that we must 

 seek the determining cause of the individual development or 

 reintegratiou. So likewise the determining causes of the 

 mutability of species, and of the slow formation of new and ever 

 more perfect species, must lie not in the environment, but in the 

 intimate structure of the idioplasm. 



As in ontogenic evolution the environment, in addition to 

 nutritive matters, provides a sum of stimuli favourable to the 

 development of hereditary tendencies ; so in phylogenic evolution 

 the environment provides impulses favourable to the development 

 of creative tendencies, and in measure as these develop, moulds and 

 modifies them, adapting them to the circumstances. 



It is not our task to follow Nageli in the development of his 

 theory. From the standpoint of general physiology, it suffices to 

 show that it harmonises perfectly with the principle we have 



