42 GENERAL CONCEPTIONS. 



of ancestral characters appearing in the embryo. The embryo is looked upon as 

 the representative of the actual ancestor by modification of which the adult form 

 was evolved. It is further assumed that the change of the embryo into the 

 adult type follows the same general course as the development of the remote 

 ancestor into the particular species under consideration. Speaking broadly, 

 this interpretation is undoubtedly justifiable. If it were exactly true, it would 

 be necessary only to know the embryology of an animal in order to establish 

 the evolution of its species. Experience, however, very quickly demonstrates 

 that this procedure is by no means possible, because the embryo is not a correct 

 or adequate record of the ancestral type. It is inadequate chiefly for three 

 reasons: First, because the embryo has necessities of its own, and in the course 

 of evolution embryos acquire special peculiarities by which they become adapted 

 to the conditions of their life. Such changes in organization do not correspond 

 to, but on the contrary diverge from, the inherited ancestral traits, and in so far 

 as they are present they mask or alter those structural features of the embryo 

 which represent the ancestral record. Second, because the embryos consist of 

 undifferentiated cells (q.v.). Now, the adult ancestors representing lower types 

 of organization of course had differentiated tissues, which enabled them to 

 perform the functions of adult life. One of the first things which will impress 

 itself upon the student of vertebrate embryology is that though he may find 

 at the proper stage in the embryo the organs of the body clearly developed, 

 yet, owing to the fact that they consist of relatively undifferentiated cells, they 

 are incapable, in large part, of performing the functions which they are ulti- 

 mately to assume, and the performance of which is the very object of their de- 

 velopment. This change in histological structure brings about a marked unlike- 

 ness of the embryo to the assumed ancestral type. Third, the embryo at each 

 stage of its development must be regarded as the mechanical cause of the next 

 and of all following stages. It must necessarily, therefore, have in itself pecu- 

 liarities by which it is distinguished from all other embryos. It is impossible, 

 accordingly, that all embryos should be alike. It is only necessary for the stu- 

 dent to compare embryos of various vertebrates one with another to satisfy 

 himself that they have conspicuous distinctive characteristics. When our 

 knowledge shall have grown sufficiently, we shall be able to classify vertebrates 

 by their embryos as perfectly, or perhaps even more perfectly, than we can by 

 the consideration of the adult forms. Every embryo is modified from the very 

 start away from the assumed ancestral organization, in order that its peculiarities 

 may cause it mechanically to produce the new form which has been evolved. 



In some of the invertebrate animals — as, for instance, among the hydroids 

 and jellyfishes — the law of recapitulation can be much more easily verified than 



