56 EVOLUTION AND ANIMAL LIFE 



can be, in some degree at least, handed on by inheritance to the 

 offspring. And there are to-day many Lamarckian evolu- 

 tionists. So that in our list of possible evolution factors the 

 so-called Lamarckian factor should not be omitted. And in 

 connection with it may be considered, by and large, the imme- 

 diate influence or non-influence on individuals and on species 

 of all environmental conditions; and particularly the results 

 of such influence during development. In fact the study of 

 development has come largely to be a study of the actual in- 

 fluences or factors that determine and guide growth, instead of 

 one purely descriptive and comparative as in the older days 

 of embryological study. Some of these factors are apparently 

 strictly inherent in the protoplasmic germ cells and in the 

 embryo substance: others are as obviously extrinsic or epigenetic. 

 And the determination of the relative influence and power of 

 these two sets of developmental factors and of the various 

 members of each set is one of the most eagerly worked-at 

 problems of modern biological study. 



Finally, the general term adaptation should be mentioned 

 in any list of evolution factors; although it is more usually 

 looked on, not as a factor, but as an evolution problem and 

 indeed one of the greatest of the problems. Adaptation is 

 precisely one of the things evolutionists are trying to find the 

 causes or causal factors of. But nevertheless the adaptability 

 of life stuff, its plasticity and capacity of advantageous reac- 

 tion, is, to many biologists, a fundamental fact in organic 

 nature, like gravitation or chemical affinity in inorganic nature: 

 a thing basic and inexplicable, and in itself a factor whose con- 

 sequences are to be determined but not further to be ques- 

 tioned as to their cause. 



