160 OUTLINES OF EVOLUTIONARY BIOLOGY 



the later part of the laying period the same parents were kept 

 under normal conditions and yielded 319 eggs from which 61 

 normal beetles were obtained and none of the other forms, and 

 these normal beetles continued to breed true for three generations, 

 after which they were killed. 



The two specimens of the immaculothorax form obtained in 

 the earlier part of the experiment unfortunately died from 

 disease, as also did all but two of the pallida. The remaining 

 two, however, both being male, were crossed with normal 

 females, yielding hybrid offspring with the decemlineaia charac- 

 ters dominant, and these hybrids, breeding inter se, gave off- 

 spring which separated out in a characteristic Mendelian fashion 1 

 into pallida, decemlineaia and hybrids again. There can be no 

 question therefore that the pallida characters, first due to modi- 

 fication of the germ cells by the action of changed environment, 

 were strictly heritable. 



It should be observed that the forms pallida and immaculothorax 

 also occur occasionally, but rarely, in a state of nature as sports 

 or mutations, a fact which suggests that sports or mutations in 

 general may owe their existence to the apparently direct action 

 of the environment upon the germ cells. It is, of course, possible, 

 or even probable, that the change in the environment merely acts 

 as a kind of liberating stimulus, whicli enables characters already 

 latent in the germ cells to express themselves in the developing 

 organism, which, under normal conditions, they are unable to do. 



We shall have to return to the question of the origin of blasto- 

 genic variations in future chapters. 



1 See Chapter XIV. 



