THE ORIGIN OF VARIATIONS 231 



In some way, therefore, the germ plasm must have changed. 

 We have then the alternatives. Is there some internal, initial or 

 driving impulse that has led to the process of evolution? Or 

 has the environment brought about changes in the germ plasm? 

 We can only reply that the assumption of an internal force puts 

 the problem beyond the field of scientific explanation. On the 

 other hand, there is a small amount of evidence, very incomplete 

 and insufficient at present, to show that changes in the environ- 

 ment reach through the soma and modify the germinal material " 

 (T. H. Morgan, Heredity and Sex, p. 17-18). 



The origin of adaptations thus, specifically in our group of 

 Crustacea, is still difficult to explain. Some advance of a sure 

 kind has nevertheless been made. We know that a funda- 

 mental property of protoplasm is its power to vary, to adapt 

 itself to changed conditions of environment. In higher animals 

 the somatic protoplasm certainly exhibits this property and 

 there is no a priori reason why the germ plasm also, should not 

 possess it. We know also that changes in one organ bring about 

 compensatory or regulating changes in others, and again there 

 is no a priori reason why the germ plasm should not partake 

 in this reaction. An adaptation in our Crustacea may have 

 originated as a mutation; in the germ plasm it may have been 

 present as a simple Mendelian characteristic subject to segre- 

 gation during the maturation stages. Later it may have become 

 too deeply impressed in the germ plasm to undergo segregation 

 and became a fundamental part of the racial plasm no longer 

 subject to extinction by natural selection while the environ- 

 ment remained the same. Such an origin, especially for all of 

 the variations in a present-day phylum, and in different phyla, 

 demands time. The history of the earth as written in modern 

 geology allows some hundred millions of years for modern types 

 to have evolved, and if twenty-five mutants of a single species 

 may be experimentally produced in four years, it is conceivable 

 that 500, ooo species of animals might have been produced in one 

 hundred millions of years, since each species possesses the power 

 to vary. 



The power to vary is not possessed by all organisms in equal 



