EVOLUTION 1063 



tive of birds and mammals, is the capacity for keeping the tempera- 

 ture of the blood and body approximately constant, day and night, 

 year in and year out. It is discussed in the physiological chapter; 

 so it is enough to say that warm-blooded animals are at a great 

 advantage over the lower forms which are all cold-blooded, i.e. 

 approximating in their body-temperature to that of the surrounding 

 medium. Warm-bloodedness facilitates the metabolism of the body; 

 it tends to maintain uniformity of its rate, and economises heat- 

 energy. Some mammals are imperfectly warm-blooded ; and most of 

 these have saved the winter situation by becoming hibemators 

 (q.v.). In newborn mammals and newly hatched birds the heat- 

 regulating arrangements are not yet fully developed, and everyone 

 knows that a short exposure to cold is fatal in these cases. No doubt 



Fig. 182. 



The Statoblast or "Winter-Egg" of a Freshwater Polyzoon (Cristatella) . It 

 is surrounded by a firm envelope, from which arise numerous hooked 

 spines, adapted to effect attachment, 



the keeping of an approximately constant temperature depends on 

 structural arrangements — involving the heat-regulating centre in 

 the brain,, the innervation of muscles, of sweat-glands, and the 

 course of the blood-stream ; but thermotaxis none the less remains 

 essentially a physiological adaptation, though requiring anatomical 

 differentiations to function in its service. 



Another functional adaptation is rapid colour change (see Struggle 

 for Existence), which in many cases, though not in all, results in 

 the animal becoming inconspicuous against its natural background. 

 This rapid change of colour depends on alterations in the position 

 and state of contraction of the pigment-cells, and these alterations 

 are brought about in various ways: (a) by the direct influence of 

 light on the skin, as in the iEsop Prawn; {h) by the influence of 

 surrounding colours on the eye and a diffusing influence through 

 the nervous system, as in fiat-fishes; or [c) through the diffusion of 

 hormones (from the pituitary body) in the blood, which affect 



