328 DARWINISM AND THE PROBLEMS OF LIFE 



separated afresh, and reunited later on. Storms swept 

 over the earth. Violent hurricanes rooted up whole 

 forests. Desert-storms buried whole stretches of green 

 land under sand. Tropical heat alternated with arctic 

 cold : floods with drought. 



When animals that had long been separated came 

 together again, they were generally so different in 

 structure and appearance that they could no longer 

 cross in such a way as to mingle the characters of one 

 species with those of the other. Even minute changes 

 in the spermatozoa of the male prevent it from pene- 

 trating any longer into the ovum and fertilising it. 

 Moreover, animals are most apt to unite with those of 

 their own species ; they have a sort of racial feeling that 

 generally prevents them from crossing with animals of 

 a different form. An unfamiliar appearance or scent 

 restrains animals from mixing, and this is especially 

 true of animals that come together after a long separa- 

 tion. We know that mammals and birds that have 

 been kept in captivity for only a brief period have not 

 only no love to expect from their kind when they are 

 released, but are actually persecuted. Even the finest 

 changes in animals often act powerfully on the 

 sense-organs of their free-living fellows. 



It is not only geological alterations of the earth's 

 surface that lead to the division of animal species by 

 isolation, but organisms may be transported into regions 

 from which they cannot return to their old home and 

 their fellows. In modern times animals have often 

 enough been transplanted by men into distant localities. 

 The Porto- San to rabbit is a proof of the fact that a 



