THE RECAPITULATION THEORY 307 



animal capable of independent existence, and 

 therefore cannot correctly represent any ancestral 

 condition, an objection which applies to the 

 developmental history of many, perhaps of most 

 animals. 



Haeckel long ago urged the necessity of distin- 

 guishing in actual development between those 

 characters which are really historical and inherited, 

 and those which are acquired or spurious additions 

 to theTecord. The former he termed palingenetic 

 or ancestral characters, the latter cenogenetic or 

 acquired. The distinction is undoubtedly a true 

 one, but an exceedingly difficult one to draw in 

 practice. The causes which prevent development 

 from being a strict recapitulation of ancestral 

 characters, the mode in which these came about, 

 and the influence which they respectively exert, 

 are matters which are greatly exercising embry- 

 ologists, and the attempt to determine which has as 

 yet met with only partial success. 



The most potent and the most widely spread 

 of these disturbing causes arise from the necessity 

 of supplying the embryo with nutriment. This 

 acts in two ways. If the amount of nutritive 

 matter within the egg is small, then the young 

 animal must hatch early, and in a condition in 

 which it is able to obtain food for itself. In such 

 cases there is of necessity a long period of larval 

 life, during which natural selection may act so as 

 to introduce modifications of the ancestral history, 

 spurious additions to the text. 



If on the other hand, the egg contain within 



