DIRECT ADAPTATION, 231 
in our zoological gardens, and exotic plants which are grown 
in our botanical gardens, are no longer able to reproduce 
themselves. This is the case, for example, with most birds of 
prey, parrots,and monkeys. The elephant, also, and the 
animals of prey of the bear genus, in captivity hardly ever 
produce young ones. In like manner many plants in a cul- 
tivated state become sterile. The two sexes may indeed 
unite, but no fructification, or no development of the fructi- 
fied germ, takes place. From this it follows with certainty 
that the changed mode of nutrition in the cultivated state is 
able completely to destroy the capability of reproduction, 
and therefore to exercise the greatest influence upon the 
sexual organs. In like manner other adaptations or varia- 
tions of nutrition in the parental organism may cause, not 
indeed a complete want of descendants, but still important 
changes in their form. 
Much better known than the phenomena of indirect or 
potential adaptation are those of direct or actual adapta- 
tion, to the consideration of which we now turn our at- 
tention. To them belong all those changes of organisms 
which are generally considered to be the results of. practice, 
habit, training, education, ete.; also those changes of or- 
ganic forms which are effected directly by the influence of 
nutrition, of climate, and other external conditions of exist- 
ence. As has already been remarked in direct or actual 
adaptation, the transforming influence of the external cause 
affects the form of the organism itself, and does not only 
manifest itself in that of the descendants. (Gen. Morph. 
ii. 207.) 
We may place the law of universal adaptation at the 
head of the different laws of direct or actual adaptation, 
