222 BIOLOGY: GENERAL AND MEDICAL 



However, though the importance and hence the order 

 of development is changed, the general plan of develop- 

 ment for each organ or system of organs is quite compar- 

 able to that seen in phylogenesis. 



"When one traces the course of development of any 

 vertebrate, he finds, speaking in general terms, that 

 those fundamental characteristics more or less common 

 to all vertebrates first appear, being followed by second- 

 ary characteristics distinguishing one class from another." 

 In vertebrate embryos, however, before the develop- 

 ment reaches a certain point, distinct resemblances to 

 invertebrate forms are met, and the younger the embryo 

 is, the more it has in common with embryos in general, 

 until at the very beginning we come to the single germi- 

 nal cell which is the starting point of every embryo. 

 These facts have found expression in the statement that 

 "the ontogeny recapitulates the phylogeny." This as a 

 theory is certainly justified; as a fact it must be further 

 explained. Von Baer, in 1828, gave us the following 

 generalizations: 



1. That which is common to a large group of animals develops in 

 the embryo earlier than that which is special. 



2. From the most generalized stage, structures less generalized 

 are developed, and so on until the most special appears. 



3. The embryo of a given animal form, instead of passing 

 through the other given forms, separates itself from them more 

 and more. 



4. Therefore, the embryo of the higher forms is never like a 

 lower form, but only like its embryo. 



These principles have since been much insisted upon, 

 especially by Haeckel, who termed the law of recapitula- 

 tion the " Biogenetic law." 



The emphasis laid upon this "law" by many writers 

 has, however, given rise to some mistakes, as students 

 are apt to think that every embryo must and does show 

 its complete phylogeny in its ontogeny. 



In explaining this misconception, Romanes says: 



"Supposing the theory of evolution to be true, it must follow that 

 in many cases it would have been more or less disadvantageous 



