xix. 2 ADJUSTMENT TO ENVIRONMENT 535 



can certainly be applied to mammals. They have a life that is more 

 free than that of other groups, in the sense that they can exist and 

 grow in circumstances that other forms of life would not tolerate, 

 and they can do this because of the elaborate mechanism by which 

 their composition is kept constant. Besides the regulation of tempera- 

 ture there is also a regulation of nearly all the components of the 

 blood, which are kept constant within narrow limits. Barcroft has 

 pointed out that the achievement of this constancy has enabled the 

 mammals to develop some parts of their organization in ways not 

 possible in lower forms. For instance, an elaborate pattern of cerebral 

 activities requires that there shall be no disturbances by sudden 

 fluctuations in the blood. 



We shall expect to find in the mammals, therefore, even more 

 devices for correcting the possible effects of external change than are 

 found in other groups. Besides means for regulating such features as 

 those mentioned above we shall find that the receptors are especially 

 sensitive and the motor mechanisms able to produce remarkable 

 adjustments of the environment to suit the organism, culminating in 

 man with his astonishing perception of the 'World' around him and 

 his powers of altering the whole fabric of the surface of large parts of 

 the earth to suit his needs. 



Such devices for maintaining stability often take peculiar and 

 specialized forms in particular cases. The activity and 'enterprise' of 

 mammals has led many of them to make use of particular structures 

 and tendencies in order to develop very odd specializations, which en- 

 able them to occupy peculiar niches. What could be more bizarre 

 than the development of the muscles of the nose until a huge mobile 

 trunk appears, so that the heaviest of four-footed beasts, while using 

 its legs for support, can also handle objects more delicately than almost 

 any other animal ? 



The mammals have developed along many special lines and many 

 of these have already become extinct; others, especially among rodents 

 and primates, remain among the dominant land-animals today. 

 Warmth, enterprise, ingenuity, and care of the young have been the 

 basis of mammalian success throughout their history. The most 

 characteristic features of the modern mammals are thus seen to be 

 largely in their behaviour and soft structures. Mammalian life is above 

 all else active and exploratory. Mammals might well be defined as 

 highly percipient and mobile animals, with large brains, warm blood, 

 and a waterproofed, usually hairy skin, whose young are born alive. 

 Since it is difficult to recognize such characters as these in fossils we 



