INTEODUCTION. 9 



due to AdaptatioD, just as we have seen that similarity is due to 

 Transmission. 



§ 9. 



Adaptation is commenced by a change in the function of organs^ so 

 that the physiological relations of organs play the most important 

 part in it. Since adaptation is merely the material expression of 

 this change of function^ the modification of the function as much as 

 its expression is to he regarded as a gradual process. As a rule, 

 therefore, Adaptation can be perceived by its results only in a long 

 series of generations, while transmission can be recognised in every 

 generation. Although Adaptation as a process cannot be directly 

 observed, it is nevertheless possible to infer it with certainty by 

 comparison. When, for example, we find a simple structure of the 

 stomach in the Carnivorous Mammalia, and, on the other hand, a 

 more complicated one in the Herbivora, and especially in those 

 which take in large quantities of food, as, for example, the Eumi- 

 nantia, we are entitled to consider the comphcation in the structure 

 of these stomachs as a change caused by the food — as an Adaptation 

 to the mode of nutrition ; and when, further, the ontogeny of the 

 Euminantia shows us a form of stomach which is simple in the early 

 stages of development, and is gradually converted into the more 

 complicated condition, ontogeny confirms the supposition we have 

 already gained by comparison. In many cases the influence of 

 Adaptation on the organisation can be observed also directly ; as, for 

 example, in many Amphibia, where the branchite which are de- 

 veloped during the larval stage are retained in function for an 

 extended period, if the opportunity of escaping from the water does 

 not arrive ; and in others, again, where the branchiae undergo 

 atrophy, as soon as their aquatic life has been exchanged for one on 

 land, although their nearest allies live in the water, and always 

 retain the branchige. In the one case we see development, and in 

 the other atrophy, as phsenomena resulting from Adaptation. 



In Adaptation, the closest connection between the function and 

 the structure of an organ is thus indicated. Physiological functions 

 govern, in a certain sense, structure ; and so far what is morpho- 

 logical is subordinated to what is physiological. This dependence 

 of the form of an organ on its activity is seen in the most elementary 

 way in the matter of size. When the function is increased, there is 

 an increase in the size of the organ. The muscular system shows 

 to what extent increase of activity affects size. Without exercise 

 the muscles undergo degeneration, till they completely disappear. 

 If they are kept in exercise, and if the demands on them are 

 increased, they develop to a considerable size. The amount of 



