HOW ANIMALS ADAPT THEMSELVES 27 



vironment." Scientific men are busily hunting 

 for instances of variation of this sort. Take for 

 example, an animal which lives sometimes in salt 

 water, sometimes in water that is only brackish ; 

 there are cases in which small differences can be 

 noticed, according to the difference in the habi- 

 tat. Notice the marine shell-fish, for instance, 

 near the estuary of a river : they are often less 

 robust specimens than are found at a point free 

 from the influence of fresh water. 



Not until the effect of known causes on the 

 rise of variations has been studied much more 

 fully than at present, will it be possible to judge 

 regarding the nature of those variations which 

 appear to be spontaneous ; for which, at present, 

 no predisposing cause can be assigned. 



A very large number of variations, however, 

 fall into the class of " Atavistic " variations ; 

 that is to say, those which show a return to 

 an ancestral type. These are variations which 

 are very rarely welcome. If, for instance, a boy 

 has a pair of handsome black rabbits, he is not 

 much pleased to find among their progeny, every 

 now and then, one of the colour of the original 

 wild Bunny. The probability, in this case, is 

 that the atavistic variety will find its way into a 

 pie, instead of being kept as a pet. Equally un- 

 satisfactory to the owner, is the incorrigibly 

 savage and intractable dog or horse a reversion 

 to the mental type of an ancestor which knew 

 not the authority of a master. 



Atavistic variation often occurs when members 

 of two well-marked varieties are mated ; so that 

 in some of the offspring produced, each parent 

 seems to cancel out the more extreme character- 

 istics of the other, leaving only the character- 



