CHAPTER VII 

 MENDEL'S LAWS 



Sexual dimorphism Cross-fertilization Parthenogenesis Self-fertilization 

 The effect of conjugation Sex an adaptation Sexual characters Alternative 

 and blended inheritance Theories of the function of sex The theory of continuous 

 evolution The theory of discontinuous evolution The Mendelian theory 

 Dominance and recessiveness Unit segregation and gametic purity Exceptions 

 to the Mendelian doctrine Latency, temporary and permanent Compound 

 allelomorphs Interpretations of Mendelian phenomena. 



T 



232. f | ^HE higher animals are all dimorphic, that is, each 

 species is divided into two groups which differ widely 

 in physical and mental characters. In the one group 

 are the males ; in the other, the females. Obviously, the sexual 

 differentiation is connected with reproduction. The mental 

 peculiarities of the two sexes are such as to impel to a series of 

 actions which eventually result in a union of sperm with ovum 

 under conditions advantageous to the development of the future 

 cell-community, to which end also the physical characteristics of 

 the parents are fitted. In all species the physical and mental 

 sexual characters furnish beautiful examples of co-adaptation. 



233. Reproduction through the union of germs from two dis- 

 tinct individuals is so familiar that we are apt to accept it as 

 ' natural ' and so dismiss it from our thoughts. Conjugation, how- 

 ever, is not a necessary antecedent of reproduction. Even in the 

 higher animals it occurs only once when sperm and ovum con- 

 jugate during the production of the millions or the billions of the 

 cell-community. The somatic cells never conjugate, and the 

 germ-cells only when a new cell-community is about to be pro- 

 duced, and then, in most cases, only with a cell from another body. 

 In the case of some plants and animals, for instance Cypris reptans 

 reproduction is apparently entirely parthenogenetic, no males 

 having ever been observed. Since these organisms retain their 

 sexual organs, they are, of course, descended from types in which 

 reproduction was, at least, sometimes preceded by conjugation. 

 In aphides reproduction is parthenogenetic during warm weather 

 when food is plentiful, but the winter eggs are fertilized, Queen 



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