Adaptation to Environment 345 



sperm of practically every other marine bony fish. 

 The number of teleosts at present in existence is about 

 ten thousand. If we accomplish all possible hybridiza- 

 tions, one hundred milHon different crosses will result. 

 Of these only a small fraction of one per cent, can live 

 (see Chapter I), and it is generally the lack of a 

 proper circulation which inhibits them from reaching 

 maturity. It is, therefore, no exaggeration to state 

 that the number of species existing today is only an 

 extremely small fraction of those which can and pos- 

 sibly do originate, but which escape our notice and 

 disappear because they cannot live or reproduce. If 

 we consider these facts we realize that the mere laws 

 of chance are adequate to account for the fact of the 

 apparently purposeful adaptations; as they are ade- 

 quate to account for the Mendelian numbers. 



