THE BIOGENETIC LAW 185 



together than the lateral ribs of any leaf. Moreover, the little green 

 caterpillars require no further protection when they sit on the under 

 side of a leaf ; they might then very easily be mistaken in toto for 

 a leaf-rib. Thus it is certainly not natural selection tuhich effects the 

 shu7iting bach of the neiv characters. Nor can this be caused by the 

 fact that the new character can only be developed gradually and 

 in several stages, for the oblique striping at any rate arises in the 

 ontogeny all at once. There must therefore be some mechanical 

 factor in development to which is due the fact that characters 

 acquired in the later stages are gradually transferred to the younger 

 stages. But this shifting backwards can be checked by the agency 

 of natural selection as soon as it becomes disadvantageous for the 

 stage concerned. 



It is in this way that I explain the fact that the majority of the 

 caterpillars of the Sphingidae are absolutely without markings when 

 they emerge from the ^gg. Thus, for instance, the caterpillars of Chwro- 

 cartipa (Fig. ii6, A),oi Macroglossa (Fig. 115), and of Deile2:>hila (Fig. 

 118, A), as well as those oi the Smerlnthus species, are at first without 

 stripe or mark of any kind ; they are of a pale green colour, almost 

 transparent, and very difficult to recognize when they sit upon a leaf. 

 How very greatly the different stages can he independently adapted 

 to the different conditions of their life, when that is necessary for 

 the preservation of the species, is shown in the most striking manner 

 by many species. Thus the little green caterpillar of Aglla tan, 

 when it leaves the egg, bears five remarkable reddish rod- like thorns, 

 which in form and colour resemble the bud-scales of the young 

 beech-buds among which they live, and which disappear later on; 

 the full-grown caterpillar shows nothing of these, but is leaf-green, 

 marked with oblique stripes. Even if the use of these reddish 

 thorns be other than I have indicated, we have in any case to deal 

 with a special adaptation of one, and that the first caterpillar-stage, 

 and what can happen at this stage is possible also at every other. 

 Nor is it only animals which undergo metamorphosis that can exhibit 

 independent phyletic variation at every stage, but those also with 

 direct development, and indeed, in the case of these, we may assume 

 adaptation of this kind at almost every stage in the history of the 

 organs, as we have already seen, because the great abridgement of tlie 

 phylogeny into the ontogeny necessitates a very precise nuitual 

 adaptation of the organ-rudiments and of the diverse rates of 

 development. 



We have thus been led by the facts discussed— and nuinenjus 

 others from other groups in the animal kingdom might be ranked 



