322 Mr. E. Ray Lankester on the 



classifications in vogue before the recognition of the origin of 

 organic forms by descent may be regarded as unconscious 

 attempts to answer the questions above put before they had 

 been rightly formulated. 



The chief means which the naturalist at present possesses of 

 making out the genealogical tree of the animal kingdom lie 

 in the fact that the individual animals living at the present day, 

 in the process of reproduction, revert to the original simple 

 condition (or nearly so) from which they have in the course of 

 long ages been evolved as specific forms. The doctrine of 

 evolution teaches us that at a certain period in the history of 

 this planet such albuminoid substances as protoplasm came, by 

 gradual building-up, into existence. From such protoplasm, 

 by slow continuous development, due to its properties of heredity 

 and adaptation, all living forms have proceeded by direct 

 descent. Strangely enough, a simple spheroid of protoplasm 

 (nucleated or not) is the form under which the detached repro- 

 ductive particle of each living organism makes its appearance, 

 and from such a spheroid every individual living thing has 

 been more or less directly developed within the space of a few 

 days or weeks. In passing from this simple condition to its 

 adult form the individual goes through a series of changes, which 

 are now explained by what may be termed ''the recapitulation 

 hypothesis," which supposes that the individual organism 

 in thus developing repeats more or less completely the successive 

 series of forms Avhich its ancestry has presented in the course 

 of past ages ; in fact the development of the individual is an 

 epitome of the development of the species. This tendency to 

 recapitulate, which is the fullest expression of the phenomenon 

 termed heredity, is liable to be masked in its effects in two 

 chief ways, due to adaptation — namely, the tendency to develop 

 directly to the adult form without exhibiting any ancestral 

 phases, and the tendency to develop evanescent organs for the 

 temporary wants of the young organism. The discrimination 

 of the appearances due to these distinct factors is the task of 

 modern embryology. It is clear that in proportion as this can 

 be effected we have in our hands in the recapitulation hypo- 

 thesis the means of determining the pedigree of all organisms. 



Comparative anatomy (the morphology of adult organisms), 

 so far as it establishes identity of structure in certain groups 

 of organisms, widens the significance of a developmental history 

 worked out in one member of such a group, and fm-nishes 

 suggestions of the highest value in the disentanglement of the 

 hereditary and adaptational factors of such a history. 



The remains of extinct forms have a specially suggestive 

 value ; but palaeontology as a whole, taken in connexion with 



