ANIMAL LIFE AND EVOLUTION 341 



acquirement and adaptation, and the handing on of it by in- 

 heritance, resulting in a rapid cumulation of difference, was 

 the basis of Lamarck's explanation of evolution. 



But owing primarily to the keenly critical examination of 

 the actual conditions and results of heredity by August Weiss- 

 mann, present-day biologists believe that such acquired char- 

 acters cannot be inherited, and hence that the modification 

 and adaptation of species has all to depend for its beginning 

 on favorable chance congenital variations, to be preserved 

 and slowly cumulated by the action of natural selection. 

 This belief makes it hard to gain a satisfactory conception of 

 the actual methods of evolution, because it makes an enormous 

 demand on variation, the precise working of selection, and the 

 extent of geological time. So that there is a strong tendency 

 among present-day biologists to search for other factors of 

 species modification, and to try to determine by experiment 

 the actual outcome in heredity of the modification of the body, 

 and, if possible, of its germ cells, by external influences. As 

 yet none of these experiments has given any sound basis for a 

 reinstatement of the belief in the inheritance of acquired 

 characters, but some of them do prove that the germ cells of 

 animals can be directly influenced by external conditions, and 

 that young, differing markedly from their parents, are produced 

 by these influenced germ cells. That is, environmental 

 influences which may modify the body may also modify the 

 germ cells and thus cause change from the species type in the 

 offspring which can be handed on by inheritance to their own 

 progeny, but these changes will not be replicas of the acquired 

 variations of the parents. Thus the only result on heredity 

 of a better exercise of an animal's capacities will be to give its 

 offspring more vigor and a better start in life perhaps, but it 

 cannot endow the offspring with the parents' actual gain in 

 fitness. Education can be handed on only by tradition, not 

 by true germinal inheritance. 



Another special subject in connection with heredity which 

 should be mentioned in this book because of its importance in 

 the work of the breeder, is that of the alleged advantage or 

 disadvantage of in-breeding. In the light of the Mendelian 



