76 THE EVOLUTION THEORY 



is to be attempted at all, it can only be through natural selection, 

 and as we have assumed once for all that our world does admit of 

 explanation, we may say, these instincts have arisen through natural 

 selection. 



Even though it may be difficult to think out in detail the process 

 of the (j-radual oripfin of such an instinctive activity, exercised once 

 in a lifetime, such as that, for instance, which impels the caterpillar 

 to spin its inti-icate cocoon, which it makes only once, without ever 

 having seen one, and thus without being able to imitate the actions 

 which produce it, we must not push aside the only conceivable 

 solution of the problem on that account, for then we should have to 

 renounce all hope of a scientific interpretation of the phenomenon. 

 We may ask, however, whether there is not something lacking in our 

 present conception of natural selection, and how it comes about that 

 useful variations always crop up and are able to increase. 



But if we must explain, through natural selection, such complex 

 series of actions as are necessary to the making of the cocoon of the 

 silkworm or of the Saturnia moth {^atiirnla carpini), what reason 

 have w^e for not referring other instincts also to selection, even if 

 they be repeated several times, or often, in the course of a lifetime? 

 It is illogical to drag in any other factor, if this one, which has been 

 proved to operate, is sufficient for an explanation. 



Thus, as far as instincts are concerned, there is no necessity 

 to make the assumption of an inheritance of functional changes, 

 any more than there is in regard to any purely morphological 

 modifications. As the instincts only exercised once show us that 

 even very complicated impulses may arise without any inheritance 

 of habit, that is, without inheritance of functional modification, so 

 there are among purely morphological characters not a few which, 

 though effective, are purely passive, which are of use to the organism 

 only through their existence, and not through any real activity, so 

 1 that thev cannot be referred to exercise, and therefore cannot be due 

 to the transmission of the results of exercise. And, if this be the 

 case, then transformations of the most diverse parts may take place 

 without the inheritance of acquired characters, that is, of functional 

 modifications, and there is no reason for dragging in an unproved 

 mode of inheritance to explain a process Mdiich can quite well be 

 explained without it. For if any part whatever can be transformed 

 solely through natural selection, why, since there is general variability 

 of all parts, should this be confined to the passive organs alone, when 

 the active ones are equally variable, and equally important in the 

 struggle for existence ? 



