THE BIOGENETIC LAW 185 



together thau the latei-al 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 eertaiitly not natural selection which effects the 

 sfi untin<j ha de of tJie new clmracfers. 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 mechani cal 

 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 shiftiag backwards can be checked by the agency 

 of natural selection as soon as it becomes disadvantajreous for the 

 stage concerned. 



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

 caterpillars of the Sphingidse are absolutely without markings when 

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

 campa (Fig. ii6, A), of ^Faeroglossa (Fig. 115), and of Beilephila (Fig. 

 118. J,\as weU as those of the Smerinthas 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 difterent stages can he independently adapted 

 to the diflerent conditions of their Kfe, when that is necessary for 

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

 by many species. Thus the little green catei-pillar of Aglia tau, 

 when it leaves the egg, bears live 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 iirst caterpiUar-stage, 

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

 Xor is it only animals which undergo metamorphosis that can exhibit 

 independent phyletic v€iriation 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 the 

 phylogeny 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 groups in the animal kingdom might be ranked 



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