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 defmiteness and constancy of species, a 



matter of daily observation in the case of man and 



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