This form of incongruence is always connected with unequal divergence between the two stages of the one systematic group--in this case the _Terebrantia_. The larvæ of this imaginal group partly possess caterpillar-like (_Phytospheces_) and partly maggot-formed (_Entomospheces_) larvæ, and differ from one another to a considerably greater extent than the saw-flies from the ichneumons.[211] The final cause of the incongruence lies therefore in this case also in the fact that one stage has suffered stronger changes than the other, so that a deeper division of the group has occurred in the former than in the latter. The analogous incongruences in single families of the Lepidoptera may have arisen in a similar manner, as has already been more clearly shown above; only in these cases we are as yet unable to prove in detail that the larval structure has become more strongly changed through special external conditions of life than that of the imagines. In the smallest systematic group--varieties, it has been possible to furnish some proof of this. The one-sided change here depends in part upon the _direct action_ of external influences (seasonal dimorphism, climatic variation), and it can be shown that these influences (temperature) acted only on the one stage, and accordingly induced change in this alone whilst the other stage remained unaltered. It has now been shown--not indeed in every individual case, but for each of the different kinds of incongruence of form-relationship--that there is an exact parallelism corresponding throughout with the incongruence in the conditions of life. Wherever the forms diverge more widely in one stage than in the other we also find more widely divergent conditions of life; wherever the morphological systemy of one stage fails to coincide with that of the other--whether in the extent or in the value of the groups--the conditions of life in that stage also diverge, either more widely or at the same time within other limits; whenever a morphological group can be constructed from one stage but not from the other, we find that this stage alone is submitted to certain common conditions of life which fail in the other stage. The law that the divergence in form always corresponds exactly with the divergence in the conditions of life[212] has accordingly received confirmation in all cases where we have been able to pronounce judgment. Unequal form-divergences correspond precisely with unequal divergence in the conditions of life, and community of form appears within exactly the same limits as community in the conditions of life. These investigations may thus be concluded with the following law:--In types of similar origin, _i.e._ having the same blood-relationship, the degree of morphological relationship corresponds exactly with the degree of difference in the conditions of life in the two stages. With respect to the question as to the final cause of transformation this result is certainly of the greatest importance.