172 LECTURES ON BIOLOGY 



frequently helps them to escape from their natural enemies. 

 Locusts in captivity will slowly and with the utmost indifference 

 chew off legs, the ovipositor, and even the abdomen. One 

 might be inclined to think that the locusts are cleaning and 

 polishing their legs, whilst in truth they are busily engaged in 

 eating them up. What induces the insects to perform this 

 incomprehensible operation we do not know, but it has been 

 observed that if they have once tasted their own flesh it is 

 impossible to induce them to take any other food. 



Another kind of self-mutilation observed in locusts and some 

 beetles is the ejection of blood, a process which is chiefly employed 

 by them as a means of defence against enemies. By the violent 

 contraction of the abdominal muscles the blood-pressure in the 

 body is increased to such an extent that the epidermis ruptures 

 in certain places and permits a thin stream of blood to be ejected 

 against the attacker. The most perfect instance of this is 

 supplied by the Algerian locust (Eugaster) which is able to 

 eject its blood with wonderful accuracy up to a distance of 50 

 centimetres. As the blood contains substances with a pungent 

 smell, their enemies, usually lizards, as a rule refrain from making 

 a second attack. 



If we finally recall the fact that many worms fall into pieces, 

 each of which may once more grow into a fully developed indi- 

 vidual, that starfishes can sever their arms by constriction, and 

 sea-cucumbers on receiving a stimulus expectorate their intestines 

 without apparently receiving any fatal injury from this horrible 

 mutilation, no one will doubt the great biological importance of 

 this adaptation. 



We have already seen in what manner Darwin's theory of 

 selection attempted to explain the origin of useful adaptations 

 and the formation of new organic species. It now only remains 

 for us to examine whether this theory is able to fulfil all that it 

 promises. Keceived first with astonishment, then with bound- 

 less enthusiasm, the doctrine of this great naturalist made in 

 a short time a veritable triumphal journey round the world, 

 and there was a time when the great majority of prominent 

 zoologists were all convinced Darwinists. The doctrine of natural 

 selection seems, indeed, exceedingly clear, nor is there any other 

 theory which vouchsafes such uniformity of observation. It is 



