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of variation in vtero, and the different means by whicli the food 

 supply of the embyro was assured were good examples of varia- 

 tions which natural selection would readily deal with. 



Darsvin and Wallace having proved that species were not 

 special creations^ and shown the extreme importance of natural 

 selection, it became possible to formulate a theory of evolution. 

 The Darwinism theory of evolution is simply an extension of the 

 theory of the origin of species. 



So far the mode of action of natural selection as the result of 

 the struggle for existence alone had been investigated, but for a 

 complete exposition of evolution an examination into the nature 

 of heredity and variability was essential. 



Darwin attempted an explanation of heredity by means of 

 his gemmule theory or pangenesis. The causes of variation, 

 except in so far as they resulted from heredity combined with 

 sexual reproduction, he did not attempt to solve. 



Pangenesis has not met with much support, chiefly because 

 it is based on pure assumption of the existence of bodies 

 (gemmules) which have never been and from their nature 

 probably never could be demonstrated. 



In spite of attempts by various investigators to modify this 

 theory it has failed to become a working hypothesis. 



Naegali and Weismann have attempted a solution of the 

 problem from an entirely different direction, and their theories 

 have been founded on demonstrable facts, each interpreting the 

 same facts in his own way. 



The facts on which these theories rest are briefly these. — The 

 presence in the reproductive cells of particles of protoplasm differ- 

 ing in their nature (as shown by their re-actions to dyes, &c.) 

 from the general protoplasm of the cell. These particles are called 

 chromatin rods, or threads. They undergo peculiar changes 

 before and after fertilization of the ovum. 



A portion is destroyed, but a portion remains, which by 

 fusing with a similar portion from another individual forms the 

 starting point for the development of the off-spring. The 

 chromatin rods are found inboth male andfemalereproductivecells. 



In unicellular forms of life the reproductive cell is identical 

 with the individual. 



According to Naegali these chromatin rods vary independently 

 of any outside circumstances. According to Weismann they can 

 only vary as the result of some outside stimulus and he regards 

 natural selection as such a stimulus. It is obvious that Weismann 

 uses the term natural selection in a very different sense to Darwin. 



