364 



THE EVOLUTION THEORY 



trb 



uij 



U12 



IU3 



of the suctorial proboscis of tlie butterliy down to the minutest 

 (k'tails. It must be these alone which cause the suctorial proboscis 

 to develop, and in some cases to de<;-enerate again, without l)rino-ino; 

 about an\- eliange in the correspondino; 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 l)utterfl3'. 

 Yet the two cases are exactl}- alike, and it would be a mistake to 

 say that the first primordium of the \ving 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 embrj^o, the ' primitive 



wing-cell,' which, how^ever, has not 

 as yet been demonstrated. But it 

 is nevertheless an integral part of 

 the caterpillar, which could neither 

 be w^anting, 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 w^ing peculiar to the species. This 

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

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

 and these become later the nerves and tracheas 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 specific 

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

 complicated in Lepidoptera. Almost every little spot and stripe 

 of the latter is handed down Avith 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 plumicornis). K, head. 

 Th, thorax, ui, inferior imaginal disks. 

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

 ni^, the primoi'dia of the limbs, oi'^ 

 and or\ the primordia of the wings and 

 ' balancers.' g, brain, bg, chain of ventral 

 ganglia with nerves which enter the 

 imaginal disks, trb, tracheal vesicle. 

 Enlarged about 15 times. 



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