152 THE EVOLUTION THEORY 



effectively adjusted to its end when the animal is under natural 

 conditions. Its specialization has its limits, and in this lies the 

 reason of its limited purposiveness. For instance, if the larva of 

 Sitavis were not impelled by the sight of every bee to spring on 

 it and cling to it, but only by the females, then many of them would 

 be saved from the fate that awaits them if they attach themselves to 

 male bees, which make no nest, or even to other Hying insects, in 

 which case also there is no possibility of further development. But both 

 these things happen, although the latter has not yet, to my knowledge, 

 been recorded of Sitaris, but only of its relative, the larva of Jfeloe. 



' Instinct goes astray,' it is often said ; but in truth it does not 

 go astray, but is only not so highly specialized in relation to the 

 liberating stimulus of the action as seems to us necessary for perfect 

 purposiveness. But in this very imperfection there lies, as it seems to 

 me, another proof that we have to do with the results of a process of 

 selection, for it is of the very nature of these never to be perfect, but 

 only relatively perfect, that is to say, just as perfect as is necessary 

 to the maintenance of the species. At the moment at which this 

 grade of perfection is reached every possibility of a further 

 increase in the effectiveness of adjustment to the end ceases, because 

 it would then no longer directly further the end. Why, for instance, 

 should the liberating stimulus in this case be more highly specialized, 

 since enough of the Sitaris larvae already succeed in attaching them- 

 selves to female bees ? It is not for nothing that the beetles of this 

 family are so prolific ; what is lacking in the perfection of the instinct 

 is made up for by the multitude of young larvae. A single female of 

 the oil-beetle (Melue) lays several hundred eggs. 



In speaking of the animal as a machine, it must be added that it 

 is a machine which can be altered in varying degrees, which can 

 be regulated to work at high or low pressure, slowly or quickly, finely 

 or roughly. This regulating is the work of the intelligence, the limited 

 ' thinking-power/ which must be ascribed to the higher animals in 

 a very considerable degree, but which, in the lower animals becomes 

 less and less apparent, until finally it is unrecognizable. That 

 instinctive actions can be modified or inhibited by intelligence and 

 will is proved by any trained beast of prey which masters its 

 hunger and the impulse to snap at the piece of flesh held before 

 it, because it knows that if it does not control itself painful blows 

 will be the consequence. In a later lecture I shall return to the con- 

 nexion between will and instinct ; all that concerns us here is to 

 regard instincts as the outcome of the processes of selection, and as an 

 indirect proof of the reality of these. 



