No. 579] PROGRESSIVE EVOLUTION 



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heterogeneous, being composed of chromomeres or ids, 

 which in their turn are composed of determinants. 



All this complexity of structure may be attributed to 

 the effects of oft-repeated amphimixis, a view which is 

 supported in the most striking manner by the fact that 

 the nucleus in all ordinary somatic cells (in animals and 

 in the diploid generation of plants) has a double set of 

 chromosomes, one derived from the male and the other 

 from the female parent, and by the well-known phe- 

 nomenon of chromatin reduction which always precedes 

 amphimixis. 



When we approach the problem of heredity from the 

 experimental side we get very strong evidence of the 

 existence in the germ-plasm of definite material sub- 

 stances associated with the inheritance of special char- 

 acters. Mendelian workers generally speak of these 

 substances as factors, but the conception of factors is 

 evidently closely akin to that of Weismann's hypothetical 

 determinants. The cytological evidence fits in very well 

 witli the view that the factors in question may be definite 

 material particles and it is quite possible that such par- 

 ticles may have a specific chemical constitution to which 

 their effects upon the developing organism are due. 



From our point of view the interesting thing is the 

 possibility that arises through the sexual process of the 

 permutation and combination of different factors de- 

 rived from different lines of descent. A germ-cell may 

 receive additions to its collection of factors or be subject 

 to subtractions therefrom, and in either case the result- 

 ing organism may be more or less conspicuously 

 modified. 



By applying the method of experimental hybridization 

 a most fruitful and apparently inexhaustible field of re- 

 search has been opened up in this direction, in the de- 

 velopment of which no one has taken a more active part 



There cannot be the slightest doubt that a vast number 

 of characters are inherited in what is called the Mende- 



