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 .:,. toto for 

 a leat-rib Thus it is certainly not natural selection which effects the 

 fftmg hack of the neiv characters. Nor can this be caused by the 

 tact that the new character can only be developed gradually and 

 m several stages, for the oblique striping at any rate arises fn the 

 ontogeny all at once. There must therefore be some mechanical 

 factor m development to which is due the fact that characters 

 acquired m the later stages are gradually transferred to the younger 

 stages But his shifting backwards can be checked by the ac^ency 

 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 Sphingid^ are absolutely without markings when 

 they emerge from the egg. Thus, for instance, the caterpillars of Ghcero- 

 carapa (Fig. ii6, A) of Macroglossa (Fig. 05), and of DeUephUa (Fig. 

 118, A), as well as those of the Smerinthus species, are at first without 

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

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

 wT ""''yj''^^^^ ^^^'' '^'^^'^^^^ stages cccn he independently adapted 

 to the diftei^nt conditions of their life, when that is necLary for 

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

 by many species. Thus the little green caterpillar of Iglla tan 

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

 whicn in form and colour resemble the bud-scales of the youn^ 

 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 han 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 histoiy of the 

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

 lA^Aogeny into the ontogeny necessitates a very precise mutual 

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

 development. 



We have thus been led by the facts discussed-and numerous 

 others from other g,-oups in the animal kingdom might be ranked 



