MATERIALS OF EVOLUTION 49 



from the other member it is easy to see why sooner 

 or later the result is disastrous owing to a conflict of 

 competing factors. 



The egg's cytoplasm that has been formed under 

 the dual influence of the maternal set of chromo- 

 somes appears to determine the early stages of de- 

 velopment so that even if the sperm introduces 

 factors that w^ould act disastrously on these stages 

 their influence does not at first show itself, but as 

 development proceeds the influence of the paternal 

 chromosomes comes more and more into play and 

 further progress is arrested. This is, in fact, what is 

 seen, in a way, when widely different species of 

 echinoderms or of fish are crossed. The earlv stages 

 of cleavage run smoothly and follow the maternal 

 type, but as the embryo develops further, irregulari- 

 ties and delays occur that bring progress to an end. 



Thus while some advance has been made in the 

 study of infertility between species, the other prob- 

 lem, namely the sterility of the hybrids, when such 

 are produced between species not too different, is 

 not so well understood, but in some cases at least 

 the immediate cause of the sterility has been found. 

 It has been shown, in recent vears, to result from 

 changes that take place in the ripening of the germ- 

 cells. There comes a time when the pairs of chromo- 

 somes unite throughout their length and subse- 

 quently separate to go into sister cells. This is the 

 so-called conjugation process. Now in certain hy- 



