GENETICS 125 



involved by the above passages. We may say that the evolu- 

 tion of the Ccelomata in order to perfect an indispensable 

 physiological and psychological endowment required long lines 

 of ancestry, and, as time alone can do nothing, we may draw 

 the further inference that the ancestral activities must have 

 been mainly of a nature to produce the necessary symbiotics. 



" From a causal-mechanical point of view," says Klebs, 

 "it is not a priori conceivable that a species can ever become 

 changed into another so long as external conditions remain 

 constant," and an Italian proverb says perhaps more aptly 

 still: " II lungo giorno no, ma il cuor fa 1'opera." 



If we add the bio-economic to this causal-mechanical 

 point of view, we may state with more precision that no 

 species can ever be changed progressively except symbio- 

 genetically, and that every retrogressive change has its cause 

 primarily in lapse of symbiogenesis. 



It was one of the Coelomata's conditions of existence and 

 of subsequent success to have had bequeathed to them an 

 ancestrally acquired physiological capital of the most ample 

 and auspicious kind. 



To what an extent could the nine Phyla rely upon the 

 ancestral dynamics bequeathed to them by the common verte- 

 brate ancestor of the Ccelomata group? To what extent did 

 any of them subsequently abuse their inheritance? These are 

 questions which we might ask, and which it might be profit- 

 able to go into some day in a systematic manner. Here we are 

 only concerned with the general principle, and to a certain 

 extent the questions raised answer themselves by the explana- 

 tions of the causes of biological success or failure already 

 given in the previous chapter. 



We have seen that Prof. Poulton felt constrained to 

 express wonderment at the fact of the "force of heredity" 

 outlasting vast periods of geological time. We argued that 

 time alone can do nothing, and we demurred to the use of the 

 term " hereditary force " as too vague. We concluded that 

 to evolve a satisfactory progenitor of subsequent successful 



