64 STUDIES IN EVOLUTION 



VI. Prolonged development under conditions favorable 

 for multiplication. (Bi.) 



The prolonged development or existence of a stock under 

 favorable conditions for multiplication may be considered as 

 one of the primary influences favoring the production of 

 spines. This implies abundance of nutrition and compara- 

 tively few enemies outside of other individuals of the same or 

 closely related species. Under a proper amount of increased 

 nutrition the vitality and reproductiveness of a stock are 

 raised, and, other things being favorable, it is found that the 

 stock will give expression to what has already been described 

 as free variation. Hypertrophy is also very apt to be one 

 result of abundant nutrition, so that structures of little or 

 no use may be developed, and some of them comprise certain 

 features which are often called ornamental. 



In the excessive multiplication of individuals it is evident 

 that there must be a great number of natural variations, and 

 that some of these will affect the pairing of the sexes in such 

 a manner as to accentuate and delimit certain variations. 

 Eventually there also comes a struggle for existence in 

 which favorable modifications have a decided advantage. In 

 this way it is believed that the great amount of differen- 

 tiation found in some isolated stocks has been brought 

 about. Primarily, then, a favorable condition for nutrition 

 is assumed, which is followed by excessive numerical multi- 

 plication; while the natural variations are augmented and 

 governed by the action of reproductive divergence for which 

 such conditions are favorable. Secondarily, these variations 

 are subjected to the influences of cannibalistic selection, 

 defence, offence, sexual selection, and mimicry. 



In illustration of the amount of differentiation attained by 

 a single stock under favorable conditions, the Amphipod 

 Crustaceans Gammarus and Allorchestes, found in lakes 

 Baikal and Titicaca, respectively, may again be noticed. 



In respect to the number of species, G-ammarus is very 

 sparsely distributed over the world, though in Lake Baikal 



