182 ' GENERAL PHYSIOLOGY 



several, or even within many generations ; this is evident from the 

 identity of the animal world found in the Egyptian graves with 

 that of the present. 



Heredity, therefore, represents an agency upon whieh depends 

 in phylogenetic development the preservation of peculiarities of 

 form that have once been present. 



2. Adaptation 



Adaptation, which changes form, is not so immediately apparent 

 as heredity, which maintains form. This is especially due to the 

 fact that the phenomena of adaptation usually require long spaces 

 of time for their observation, while heredity appears in every gene- 

 ration of organisms. But the results of adaptation are seen 

 daily, usually without this fact being recognised. The fact of 

 purposefulness in living nature, which was so marvellous to 

 men of science in early times, even down to the middle of the 

 present century, forced them constantly to embrace teleology, 

 i.e., the hypothesis of a fore-ordained plan of creation, such as 

 dogmatic theology, preserving faithfully the ancient venerated 

 ideas, accepts to-day. This purposefulness in nature is the simple 

 expression or, better, the result of the adaptation of organisms 

 to their vital conditions in the widest sense. 



Aquatic animals are adapted very perfectly to life in water, 

 terrestrial animals to life upon dry land, flying animals to life in the 

 air. Fishes have limbs in the form of fins, which function very 

 perfectly as rowing-organs ; terrestrial vertebrates have in place of 

 fins legs for walking and creeping upon dry land ; birds have wings 

 constructed most fittingly, with which their light bodies, supported 

 by bones containing air, soar through the air so perfectly that up to 

 the present all inventors of artificial flying machines have tried in 

 vain to imitate them. But only in single cases in the develop- 

 ment of the individual can an adaptation to other conditions be 

 traced. Thus, the larvae, of frogs, so long as they live in the water 

 as tailed tadpoles, breathe like fishes by means of gills, which are 

 constructed very simply and suitably for obtaining from the 

 water the air dissolved in it. As soon as the small frogs come 

 to the land, the tails shrink, the gills degenerate, and the 

 lungs develop, by means of which, like all terrestrial animals, 

 they take air directly into their bodies. If the tadpoles be 

 prevented artificially from creeping upon dry land, they retain 

 their tail and gills, and the lungs do not develop even though 

 the animals reach a considerable size. Such examples prove 

 that all organisms are adapted very fittingly to their vital 

 conditions; and the later zoological and botanical investigations 

 have shown that these adaptations extend frequently to the mi- 

 nutest details, of which an untrained observer would never think. 



