364 



THE EVOLUTION THEOKY 



luj 



IU3 



of the suctorial proboscis of the butterfly down to the minutest 

 details. It must be these alone which cause the suctorial proboscis 

 to develop, and in some cases to degenerate again, without bringing 

 about any change in the corresponding parts in the caterpillar. 



This example seems to me to be preferable to that of the wings 

 of insects in this respect, that there is no organ in the caterpillar 

 with a specific function corresponding to the wing of the butterfly. 

 Yet the two cases are exactly alike, and it would be a mistake to 

 say that the first primordium of the wing within the caterpillar 

 is not a part of the caterpillar at all. At first, certainly, it is only 

 a group of cells on the skin, occurring at a particular spot on the 

 dorsal surface of the second and third segments of the caterpillar, 

 and doubtless arising from a single cell of the embryo, the ' primitive 



wing-cell,' which, however, has not 

 as yet been demonstrated. But it 

 is nevertheless an integral part of 

 the caterpillar, which could neither 

 be wanting, nor be larger or smaller, 

 and so on; which, in short, does 

 mean something for the caterpillar, 

 although perhaps not more than 

 any other of the skin-cells. For 

 the butterfly, however, this area on 

 the skin means the rudiment of the 

 wing; for from it alone can there 

 arise by multiplication the aggregate 

 of cells which grows out into a hollow 

 protuberance, enlarges by degrees 

 into a disk, the imaginal disk, and 

 eventually develops into the form of wing peculiar to the species. This 

 imaginal disk is connected very early with nerves and with tracheee, as 

 may be beautifully seen especially in dipterous larvae (Fig. 89, oi), 

 and these become later the nerves and trachese of the wing, while 

 thousands of peculiar scale-like hairs develop on the upper surface; 

 in short, the rudiment becomes a perfect wing with its speciflc 

 venation, and with the marking and colouring which is often so 

 complicated in Lepidoptera. Almost every little spot and stripe 

 of the latter is handed down with the most tenacious power of 

 transmission from generation to generation, and each can at the 

 same time be transmissibly varied ; the same is true of the venation, 

 which is so important systematically just because it is so strictly 

 hereditary, yet it too can vary transmissibly, as can also the hooked 



Fig. 89. Anterior region of the larva 

 of a Midge (Corethra 'plumicamis) . K, head. 

 Th, thorax, ui, inferior imaginal disks. 

 oi, superior imaginal disks, ui', ui^, and 

 ui', the primordia of the limbs, oi' 

 and oi', the primordia of the wings and 

 'balancers.' 3, brain. 63, chain of ventral 

 ganglia vyith nerves vphich enter the 

 imaginal disks, trb, tracheal vesicle. 

 Enlarged about 15 times. 



