Chapter III: Instinctive Behaviour (3)

The net result of a study of instinctive behaviour is to lead us to the conclusion that its evolution runs parallel with the evolution of animal structure. This is perhaps best seen in the case of those insects in which typical instinctive acts are performed by larvæ of wholly different form and structure, though they are stages in the development of the same species. This is exemplified in the cases of _Sitaris_, _Argyromœba_, and _Leucopsis_ which have been briefly described. It is probable that in all cases of instinctive action natural selection has been a co-operating factor. Without going so far as to assert with Professor Weismann the “all-sufficiency of natural selection,” we may echo the words of Professor Groos,[47] and say: “Nevertheless, we know no principle except that of selection, and we must go as far as that will take us. Absolute knowledge of such phenomena is unattainable.” And in this conclusion we have the support of Dr. Peckham, who says,[48] “We have found them [the instinctive acts of solitary wasps] in all stages of their development, and are convinced that they have passed through many degrees from the simple to the complex, by the action of natural selection. Indeed, we find in them beautiful examples of the survival of the fittest.”