LAWS OF VARIATION 191 



on the whole organization or on certain parts alone, and 

 indirectly through the reproductive system. In all cases 

 there are two factors, the nature of the organism, which 

 is much the most important of the two, and the nature 

 of the conditions. The direct action of changed condi- 

 tions leads to definite or indefinite results. In the latter 

 case the organization seems to become plastic, and we 

 have much fluctuating variability. In the former case 

 the nature of the organism is such that it yields readily, 

 when subjected to certain conditions, and all, or nearly 

 all, the individuals become modified in the same way. 



It is very difficult to decide how far changed condi- 

 tions, such as of climate, food, etc., have acted in a definite 

 manner. There is reason to believe that in the course of 

 time the effects have been greater than can be proved by 

 clear evidence. But we may safely conclude that the 

 innumerable complex coadaptations of structure, which 

 we see throughout nature between various organic beings, 

 cannot be attributed simply to such action. In the fol- 

 lowing cases the conditions seem to have produced some 

 slight definite effect: E. Forbes asserts that shells at their 

 southern limit, and when living in shallow water, are more 

 brightly colored than those of the same species further 

 north or from a greater depth; but this certainly does 

 not always hold good. Mr. Gould believes that birds of 

 the same species are more brightly colored under a clear 

 atmosphere than when living near the coast or on islands, 

 and Wollastou is convinced that residence near the sea 

 affects the colors of insects. Moquin-Tandon gives a list 

 of plants which, when growing near the sea-shore, have 

 their leaves in some degree fleshy, though not elsewhere 

 fleshy. These slightly varying organisms are interesting 



