TRANSFORMISM 217 



Such are instances of the adaptations of the morpho- 

 logy and functioning of organisms consequent on 

 changes of environment. 



What is an adaptation ? The term plays a great 

 part in biological speculation, but it is often used in a 

 loose and inaccurate manner, and not always in the 

 same sense. It suggests that the organism is contained 

 by the environment, and that its form becomes 

 adapted to that of the latter, just as the metal which 

 the ironfounder pours into the mould takes the form 

 of the cavity in the sand. ' We see once more how 

 plastic is the organism in the grasp of its environ- 

 ment " such a quotation from morphological litera- 

 ture is perhaps a typical one. Over and over again 

 this passive change in the organism as the result of 

 the action of something rigid which presses upon it 

 is what is understood by an adaptation. No doubt 

 the organism may be so affected, and often the change 

 which it experiences is of the same order as the en- 

 vironmental change. In the winter many animals 

 become sluggish and may hibernate ; their heart-beats 

 slow down ; their respiratory movements become less 

 frequent, and generally the rate of metabolism, that 

 is the rapidity with which chemical reactions proceed 

 in their tissues, becomes lessened. All these changes 

 become reversed in sign when the temperature again 

 rises. The time of year at which a fish spawns de- 

 pends on the nature of the previous season. The rate 

 of development of the egg of a cold-blooded animal 

 varies with the temperature. The quantity of starch 

 formed in a green leaf depends on certain variables 

 the intensity of light, the temperature, and the 

 quantity of carbonic acid contained in the medium 

 in which it is placed. In all these cases the rate at 

 which certain metabolic processes go on in the body 



