REGRESSIVE MODES OF THE PRINCIPLES. 129 



8. Enmronal Partition. 



Emnronal partition depends on influences quite similar to those 

 producing environal isolation, except that the seasonal, cyclical, and 

 fertilizational forms are wanting. This is because these forms depend 

 on inherited characters rather than on acquired habits, while envi- 

 ronal partition is due to incompatibility in the acquired habits of 

 individuals usually belonging to groups that have been locally sepa- 

 rated for a time. Industrial and migrational partition tend more or 

 less directly to produce groups with somewhat divergent habits, w T hile 

 transportational, geological, and artificial partition open the way for 

 divergent forms of innovation, tradition, and election, to establish 

 divergent types of habitudinal groups. Moreover, these forms of par- 

 tition tend directly to produce isolation and consequently divergent 

 racial groups. 



II. THE REGRESSIVE MODE OF EACH SEGREGATIVE PRINCIPLE. 



Regressive selection may be produced either by the cessation or by 

 the reversal of a long-established form of selection. Near the end 

 of the last chapter we referred to the Old World cuckoo and the 

 American cow-bird as examples of degeneracy in the instincts for nest- 

 building, for incubation, and for the feeding of their own young a 

 degeneracy that seems to have been produced by the gradual cessa- 

 tion of the selection by which these instincts had for countless gener- 

 ations been maintained. We also found that there was reason to 

 believe that the discovery of substitutes for mother's milk is, in 

 certain races of mankind, leading to decay of the power to give suck, 

 through the survival of the children of mothers who, under the con- 

 ditions of primitive times, would have entirely failed of having any 

 share in the parentage of the next generation of parents. Examples 

 of the reversal of selection are found in the history of species that, 

 through the coming and going of the ice age, have for many genera- 

 tions been subjected to increasing cold, and then for many generations 

 to increasing warmth. 



Regressive election arises when any tradition or acquired character 

 that has long been necessary for success in a given community ceases 

 to be so. It often prepares the way for regressive selection. For 

 certain races of dogs the traditional methods of finding food are very 

 different from those that were current with their primitive ancestors, 

 and the cessation of the necessity for the strenuous life of the old 

 times has brought regressive selection, resulting in the decay of some 

 of the old instincts. 



