Apkii, 7, 1905.] 



SCIENCE. 



525 



the case where inheritance is sharply al- 

 ternative, but subject to extremely great 

 fluctuating- variations. It is open to ques- 

 tion whether blending characters of this 

 sort found in many breeds may not have 

 been created by selection from masses of 

 tiuctuating variations. It will be impor- 

 tant to know further whether or not these 

 extreme fluctuating series have had their 

 origin in mutations. Not improbably, as 

 de Vries has in part suggested, one-sided 

 variation curves indicate the occurrence of 

 mutations of this sort. 



The ^Mutation Theory From the Stand- 

 point of Cytology: Edwin G. Conklin, 

 professor of zoology, University of Penn- 

 sylvania. 



I. The mutation theory is founded upon 

 the idea that mutations are primarily 

 germinal, that they arise in one or both of 

 the sex cells and only later appear in the 

 adult organism. In contradistinction to 

 certain theories of evolution which are con- 

 cerned chiefly with the modifications of 

 adult Structures, the mutation theory is 

 primarily concerned with modifications of 

 the germ, and here it comes into direct re- 

 lation with the science of cytology. 



De "Vries tells us that the foundations of 

 the mutation theory Avere laid in his doc- 

 trine of intracellular pangenesis. Like 

 Darwin, Galton, Weismann and many 

 others, he recognized the fact that the 

 method of evolution is at bottom a problem 

 of inheritance and that, in the words of 

 Osborn, 'When we have reached a heredity 

 theory that will explain the phenomena of 

 inheritance, the method of evolution wall 

 itself be a thing of the past.' 



It seems like a mere truism to affirm that 

 the evohition of animals and plants must 

 be accompanied by an evolution of their 

 germ cells, and that the principal problem 

 of evolution is not how modifications are 

 produced in adults, but how they arise in 



the germ. And yet with few exc^eptions 

 previous theories of evolution have con- 

 cerned themselves only with the transmuta- 

 tions of adult forms and have paid no at- 

 tention to the modifications of the egg or 

 sperm or embryo. The mutaticn theory is 

 a theory of the evolution of organisms 

 through the evolution of their germ cells 

 and it is, therefore, founded primarily 

 upon cytologieal phenomena. 



An antecedent objection to any such 

 theory is the very general opinion that the 

 germ cells are composed of 'simple, undif- 

 ferentiated protoplasm' and that they do 

 not contain specific morphological elements 

 upon which evolutionary forces might act. 

 However, such a view is supported neither 

 by observation nor by the latest and most 

 careful experiments. AA^e know that the 

 cell is vastly more complex than was as- 

 sumed a few years ago, and there is no good 

 reason for supposing that all of its visible 

 structures are now known. The fact that 

 fragments of eggs may in some instances 

 give rise to entire embryos does not neces- 

 sarily imply, as is usually assumed, that 

 the egg is undifferentiated. In eggs, as 

 in adult forms, the degree of differentia- 

 tion may be largely independent of the 

 power of regeneration or regulation, and 

 certainly such experiments do not nullify 

 the most positive and direct evidence, 

 drawn from many sources, as to the com- 

 plexity of the germ. 



Extensive studies which have been made 

 upon the structure of the nucleus have 

 brought to light a degree of organization 

 in this part of the cell which was wholly 

 unexpected. It has long been known that 

 in any given species the number of chro- 

 matic threads or chromosomes in the 

 nucleus is constantly the same in all kinds 

 of cells, except in the last stages of the 

 formation of the sex cells, where the num- 

 ber is one half the normal; in the union 

 of the egg and sperm nuclei in the fertiliza- 



