138 REDUCTION OF VARIABILITY. 



limit our comparison to allogamous organisms. We will prob- 

 ably find that even in a lake of a few acres there are local forms 

 of such animals as fresh-water snails in different localities of the 

 pond. But we will find only one species of trout in it, and not 

 slightly longer animals along one bank and slightly higher 

 along the opposite bank. 



In animals with sedentary habits, we are likely to find this 

 existence of local forms. If the difference between such groups 

 is due among other genes to one which has a very decided in- 

 fluence upon the appearance of the animals, it may be possible 

 to trace the territory covered by individuals carrying this gene. 

 And we will probably find that different genes have different 

 and over-lapping territories. As, however, the action of most of 

 the individual genes is slight, and as very often several genes 

 may influence the development in analogous ways, the effect 

 will be, that from one end of the range to the other we will see 

 the animals of such a group gradually change from one local 

 form into the other. Such cases are the despair of the systema- 

 tic zoologist. For purposes of classification he will have to 

 choose between lumping all the forms into one species, or se- 

 lecting some of the most striking types as specific types. 



The facts in these cases are very much obscured by the cir- 

 cumstance that among the causes of this multiform appearance 

 of such a group, which is something very different from po- 

 lymorphy, there may be two things, first toleration of chance 

 combination of genes, and secondly real adaptation to the en- 

 vironment. When we observe a series of birds of one group, and 

 see that the individuals from the coastal marshes are darker, 

 and give place more inland to lighter and lighter birds, up to 

 very pale desert-birds, part of this diversity may be real adap- 

 tation. At the same time a corresponding diversity in the num- 

 ber of scales on the tarsus may be a case of mere chance, plain 

 toleration. And both sets of characters may be mainly due to 

 genotypic constitution, and be found to breed true in any en- 

 vironment. 



From the work of the Berkeley museum of Vertebrate Zoo- 



