30 GENERAL CONCEPTIONS. 



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 fea- 

 tures of the embryo which represent the ancestral record. Second, because the em- 

 bryos consist of undifferentiated cells (compare page n). 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 ultimately to 

 assume, and the performance of which is the very object of their development. 

 This change in histological structure brings about a marked unlikeness of the em- 

 bryo to the assumed ancestral type. Third, the embryo at each stage of its de- 

 velopment must be regarded as the mechanical cause of the next and of all fol- 

 lowing stages. It must necessarily, therefore, have in itself peculiarities 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 student to compare embryos 

 of various vertebrates one with another to satisfy himself that they have conspicuous 

 distinctive characteristics. When our knowledge shall have grown sufficintely, 

 we shall be able to classify vertebrates by their embryos as perfectly as 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 in the 

 higher forms which have purely embryonic types of development. From what has 

 been said, it will be recognized that the likeness of the embryo to the adult lower 

 form is a general morphological resemblance only, not an exact one, and that there- 

 fore it is extremely difficult to infer from the embryonic organization what the 

 ancestral type was. Hitherto all phylogenetic inferences drawn by embryologists 

 have been largely speculative in character, and, it may be added, have been more 

 remarkable for their number and variety than for their value. 



The resemblance between embryos and lower adult forms has been known for 

 a century past. It was first adequately asserted in 1811 by J. F. Meckel and 

 since then has been constantly discussed. More, perhaps, was done to emphasize 

 it by Louis Agassiz than by anyone else. Von Baer, the creator of modern 

 scientific embryology, called attention in 1828 to the limitations which must neces- 

 sarily be put upon Meckel's generalization. It is to be regretted that von Baer's 

 wise thought on this subject has not been 'more appreciated. He put forth four 



