ORIGIN OF SPECIES 215 



from the simple division of the apparently structure- 

 less speck of protoplasm, and next the cleavage of 

 the nuclear unicellular organism, through numerous 

 methods, up to the birth of the mammalian offspring, 

 and to the coming of the human child into the home 

 lovingly prepared for its advent. 



However, as mentioned before (p. 50), funda- 

 mentally all methods and forms of reproduction are 

 alike in that they represent the detachment of a part, 

 or particle, of the parent organism, or of two organ- 

 isms, which, given food supply and favorable con- 

 ditions, grows into the likeness of the parent form. 

 The close relationship that exists between nutrition 

 and reproduction is well known to all students of the 

 multitudinous phenomena of the reproduction of 

 organisms. 



Reproduction is asexual or sexual. In the lowest 

 forms of life, reproduction is asexual. In some forms 

 asexual and sexual generation alternate. In the higher 

 animals reproduction is sexual. 



Parthenogenesis — in the strict meaning of the 

 word — seems to be entirely out of the question among 

 organisms as high as the vertebrates. True, Dr. Leo 

 Loeb (whose cultures of cells in vitro have been 

 mentioned) found peculiar structures in the ovaries of 

 guinea-pigs, which structures, he said, "must be 

 interpreted as embryos developing parthenogeneti- 



