ONTOGENESIS 235 



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 

 to a developing type that it should have been obliged to reproduce 

 in its individual representations all the phases of development 

 previously undergone by its ancestry even within the limits 

 of the same family. We can easily understand, for example, that 

 the waste of material required for building up the useless gills of 

 embryonic salamanders is a waste which, sooner or later, is likely 

 to be done away with; so that the fact of its occurring at all is 

 in itself enough to show that the changes from aquatic to terrestrial 

 habits on the part of this species must have been one of com- 

 paratively recent occurrence. Now, in as far as it is detrimental 

 to a developing type that it should pass through any particular 

 ancestral phases of development, w r e may be sure that natural 

 selection or whatever other adjustive causes we may suppose 

 to have been at work in the adaptation of organisms to their 

 surroundings will constantly seek to get rid of this necessity, 

 with the result, when successful, of dropping out the detrimental 

 phases. Thus the foreshortening of developmental history which 

 takes place in the individual lifetime may be expected often to 

 take place, not only in the way of condensation, but also in the 

 way of excision. Many pages of ancestral history may be re- 

 capitulated in the paragraphs of embryonic development, while 

 others may not be so much as mentioned. And that this is the 

 true explanation of what embryologists term the ' direct ' develop- 

 ment or of a more or less sudden leap from one phase to another, 

 without any appearance of intermediate phases is proved by the 

 fact that in some cases both direct and indirect development occur 

 within the same group of organisms, some genera or families 

 having dropped out the intermediate phases which other genera 

 or families retain." 



