8o6 LIFE : OUTLINES OF GENERAL BIOLOGY 



of hare and hound, deer and wolf. Evolution is, on the whole, 

 integrative. 



In connection with accidents, we should keep in mind the wide- 

 spread occurrence of regenerative capacity, whereby the maimed 

 can be mended and the lost part replaced. This wanes in higher 

 animals which have nimble wits to save them, but it is very common 

 among lower forms of life, such as starfishes, worms, and long-legged 

 crustaceans. According to Lessona and Weismann, regeneration 

 tends to occur in those animals and in those parts of animals which 

 in the natural conditions of their life are peculiarly liable to non-fatal 

 injury. 



Thus the cham?eleon is one of the few lizards that has lost the 

 regenerative capacity and cannot re-grow its tail ; and we reasonably 

 correlate this with the fact that the tail is not usually vulnerable, 

 being kept coiled round the branch. In one starfish, at least, the 

 continuance of the race is in part secured by the habit of breaking 

 off an arm which can, if it is lucky, grow into a complete animal. 

 But our point is simply that against the frequency of accidents we 

 must balance the astonishing power of repair which many animals 

 exhibit. Even birds, with legs, wings, or even sternam broken in 

 flight, as by collision with an unnoticed telegraph wire, may 

 marvellously recover, and fly again for years after. Much more 

 important, however, is the resourcefulness and the wealth of 

 adaptations that the chances of violent death have helped to evolve. 



The second kind of death might be called intrusive, microbic, or 

 parasitic; and it is the commonest end for man and his domestic 

 animals. We have become so familiar with the frequency of microbic 

 death that we have long fallen into a fatalistic acquiescence, which 

 still retards the advance of preventive medicine. Bacteria are 

 directly responsible for plague, cholera, tuberculosis, diphtheria, 

 typhoid, and so on through the long (but probably shortening) 

 list of microbic diseases. 



But apart from the diseases due to bacteria — which seem to be 

 a kingdom by themselves, though the greedy botanist long claimed 

 them — ^there are others which are due to microscopic animals. 

 Thus there is the Plasmodium which causes malaria when intro- 

 duced into man's blood by the bite of an infected mosquito, and 

 there is the Tr3^panosome, which causes sleeping sickness when 

 injected into man by the Tsetse fly. The list of virulent Protozoa 

 continues to grow; and Protozoology is now a specialism by itself, 

 like Bacteriology. 



Besides disease-causing Bacteria and Protozoa, there are the 

 filterable viruses, implicated in such diseases as smallpox and 

 hydrophobia, which cannot be referred to any definite group. Of 

 most of them as yet we have only a physiological and an ultra- 

 microscopical knowledge. There is a difference between being 



