CHAPTER III 



THE CHEMICAL BASIS OF GENUS AND SPECIES 



I. It is a truism that from an egg of a species an 

 organism of this species only and of no other will arise. 

 It is also a truism that the so-called protoplasm of an 

 egg does not differ much from that of eggs of 

 other species when looked at through a microscope. 

 The question arises: What determines the species of 

 the future organism? Is it a structure or a specific 

 chemical or groups of chemicals? In a later chapter, 

 we shall show that the egg has a simple though 

 definite structure, but in this chapter we shall see 

 that the egg must contain specific substances and 

 that these substances which determine the "species'* 

 and specificity in general are in all probability proteins. 

 Since solutions of different proteins look alike under a 

 microscope we need not wonder that it is impossible 

 to discriminate microscopically between the protoplasm 

 of different eggs. 



The idea of definiteness and constancy of species, a 



matter of daily observation in the case of man and 



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