298 PROBLEM OF THE ORIGIN OF SPECIES. 



doctrine of the continuity of the germ plasm in its broad sense i. c, the 

 genetic continuity of germinal material as a true generalization, undemon- 

 strated, to be sure, but upheld by all of the best observational, experimental, 

 and theoretical work. The acceptance of the doctrine of the continuity of 

 the germ plasm, irrespective of how we conceive it to exist, is an absolutely 

 necessary accompaniment to the fact of the existence of heredity. With the 

 exception of Eimer and his followers, practically all biologists accept this 

 doctrine unreservedly in one form or another. Indeed, without it the most 

 ordinary facts of development and heredity become inexplicable, excepting 

 as miracles, and orderly evolution and development become inconceivable 

 processes. 



THE METHOD OF EVOLUTION. 



On the basis of our present knowledge the problem of the method of evolu- 

 tion presents two separate minor problems for consideration : ( 1 ) the origin, 

 or the cause and control of variations : (2) the preservation, or the action of 

 natural selection upon the new variations as they appear. These two 

 divisions of the main problem we shall here consider separately. 



Variation and natural selection in their various aspects furnish the entire 

 solution to the problem of the method of evolution and of adaptation, and 

 could we arrive at a solution of the course and sequence of stages in varia- 

 tion and natural selection, we should have the basis for a complete solution 

 of the problem of the origin of species. Such a solution we can not hope to 

 arrive at, at least not for a long time to come, and probably ultimately the 

 best we can do is to push back to some impassable limit the description of the 

 sequence of stages, the primal stages remaining unknown. 



The Nature, Cause, and Control of Variation. 



The phenomenon of variation primarily owes its existence to the fact that 

 community of descent and heredity tend to produce the exact counterpart of 

 the parent organism ; the process of development, however, is not carried out 

 under absolutely constant or uniform conditions, but in a world wherein there 

 exist changing environmental states in endless perplexity. This results in 

 the turning aside of the line of development from the parental standard, per- 

 haps ever so little or only in one character ; but in this we have deviation or 

 variation. 



While we can not hope to explain, excepting through philosophical specu- 

 lation, the origin of the phenomenon of variation, we do know the law or 

 sequence of events which variations follow. It has been abundantly dem- 

 onstrated in the physical and mathematical sciences that variations follow 

 precisely the law of error, and in organisms it is becoming more and more 

 certain that all variations follow most rigorously the same law. In organ- 



