ORIGIN AiXD SIGNIFICANCE OF SPINES 37 



Allorchestes have arisen from a single parent stock, within a 

 small geographic province. The natural interpretation seems 

 to be ((() that the environment is favorable, as evinced from 

 the great numljcr of individuals; (6) that this has favored 

 and increased the growth force; and (c) that, finally, the law 

 of multiplication of effects, reproductive divergence, ^^ the 

 survival of the unlike, and the conservative forces of natural 

 selection and heredity have directed the growth force, and 

 produced the specific differentiation which is now found. 



A factor of Evolution, called " Reproductive Divergence " 

 by Vernon,^^ seems to be operative here, since it affords an 

 explanation for a means of differentiation in a single stock 

 under a common environment. As this factor has but 

 recently been discussed, it may well be defined at this time, 

 so as to enable a direct application to be made. Reproduc- 

 tive divergence assumes tliat in many species there will be 

 greater fertility between individuals similar in color, form, or 

 size, than between individuals not agreeing in these respects, 

 and that in subsequent generations the divergence will 

 become progressively greater in respect to the characteristic 

 in question, so that finally the original stock will become 

 separated into distinct varieties, sub-species, or species. 



When, from any cause, the forces of nutrition are directed 

 toward spine production, and when the direct results are 

 accomplished in the reciprocal formation of one or more 

 spines, there is often an apparent inductive influence or 

 impulse given to growth toward the further production or 

 repetition of spines. This may result in the formation of 

 compound spines, or a group of sijines, or even produce a 

 generally spinous condition. 



Naturally, spines arising through growth force may be 

 useful for defence and offence, and the selective influences 

 of sex and mimicry may also tend to greater development 

 and elaboration. Furthermore, growth forces reacting on 

 any external structures, as lines, lamellae, ribs, nodes, etc., 

 may tend to differentiate such ornaments into spines. 



Therefore, under the general consideration of spines pro- 



