CHAPTER XII 

 GENERATION, SEX AND ONTOGENY 



"Unter jedem Grab liegt erne Weltgeschi elite " (German proverb). 



EACH animal, each plant, must have its individual beginning, 

 its "creation/' and its individual development from this begin- 

 ning to full grown, completely developed condition. For no 

 organism is born fully developed. Even the simplest organ- 

 isms, the one-celled kinds, whose " creation' is accomplished 

 simply by the splitting in two of a previously existing individual 

 of their kind, are not produced full-fledged. They have at 

 least to increase from half size to full size, that is, to grow, and 

 there are very few if any of them that do not have to effect 

 changes in their body structure during this period of growth; 

 that is, they have to undergo some development. The begin- 

 ning, then, is always from a previously existing organism but 

 how could it always have been? and between this beginning 

 and the normal mature or full-fledged creature there has 

 always to be some development. The beginning is called 

 generation; the development, ontogeny. 



We are all so familiar with the fact that a kitten comes into 

 the world only through being born as the offspring of parents 

 of its kind, that we shall likely not appreciate at first the full 

 significance of the statement that all life comes from life; that 

 all organisms are produced by other organisms. Nor shall we 

 at first appreciate the importance of the statement. This is a 

 generalization of modern times. It has always been easy to see 

 that cats and horses and chickens and the other animals we 

 familiarly know give birth to young or new animals of their own 

 kind; or, put conversely, that young or new cats and horses and 

 chickens come into existence only as the offspring of parents 

 of their kind. And in these latter days of microscopes and 



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