THE EPIC OF EVOLUTION 519 



swamping with parent stocks ; and finally (g) result in a newly 

 ADAPTED SPECIES. Somc brief expansion of these six points follows. 



Variation 



Darwin started out with the universally observable fact of varia- 

 tion among organisms as an axiom. Unlike Lamarck, he did not 

 make any particular attempt to find out the underlying causes of 

 variation. He pointed out, how^ever, that even things so apparently 

 alike to the casual observer as a flock of sheep invariably reveal in- 

 dividual differences to the shepherd who knows his sheep. Each 

 structural feature of an organism may exhibit variation, making 

 an enormous range of variability possible among a group of similar 

 individuals. Variations are of different kinds so far as they affect 

 the survival of individuals. Some are useful in survival, some are 

 indifferent, and some are either harmful or even lethal, "There is 

 none perfect, no not one." 



Organisms do not vary in order to become better adapted to their 

 environment, as Lamarck assumed in the case of his fantastic deer 

 that became a giraffe, but they may be better adapted to the en- 

 vironment as a result of the occurrence of variation. 



Overpopulation 



Both Darwin and Wallace were much impressed by the writings 

 of an English clergyman, Thomas Malthus (1766-1834). This math- 

 ematically inclined gentleman, who lived in the prolific days of large 

 families, was much concerned by the fact that mankind was ap- 

 parently increasing faster than the food supply. Darwin goes to 

 considerable length to point out that every organism produces many 

 more offspring than can possibly grow to maturity, even under the 

 most favorable conditions. A single toadstool, for example, may 

 easily produce a million spores, while a termite queen can furnish 

 an average of an egg a minute for a year at a time. Even the animals 

 breeding most slowly would require, if all their young succeeded in 

 becoming adult, only a few centuries of unlimited geological time to 

 overrun the entire world. Yet, in general, organisms do no more 

 than hold their own year after year, except occasionally when the 

 ''balance of nature" gets upset, and plagues of grasshoppers, star- 

 lings, weeds, gypsy moths, and what not, flare up locally for a limited 

 period. 



H. w. H. — 34 



