THE GERM-PLASM THEORY 363 



would be inclined to suppose that there was some internal connexion 

 between the colour of the caterpillar and that of the butterfly, since 

 the butterfly also occurs in two colours. It was shown, however, 

 that the colour of the butterfly "had nothing to do with that of 

 the caterpillar, for it is known to be dependent on the season, and 

 is a seasonal dimorphism, 'while the two forms of caterpillar may 

 occur side by side at all times of the year.' 



Subsequently I made a similar experiment with the dimorphic 

 caterpillars of the 'fire '-butterfly (Polyommatus phlceas), and it yielded 

 the same result. The pure green caterpillars became the same butter- 

 flies as those marked with broad red longitudinal stripes, and in this 

 case we can definitely describe both colours as protective, for the 

 green form is adapted to the green under surface of the leaf, the 

 red-striped to the green red-edged stalk of the lesser sorrel (Rumex 

 acetosella). 



There was really no necessity for special proofs that the 

 caterpillar and butterfly vary transmissibly in complete independence 

 of each other, for the facts of metamorphosis alone are enough to 

 prove it. How would it have been possible otherwise that the jaws 

 adapted for biting should, in the primitive insects, and in the locusts 

 which are nearest to them, remain as a biting apparatus throughout 

 life, while in the caterpillar they are modified during its pupal 

 stage into the suctorial proboscis of the butterfly? The parts of 

 insects, therefore, must be capable of transmissible variation in the 

 stages of life independently of each other. Not only have the jaws 

 of the leaf -eating caterpillars remained unaltered, while in the 

 sexually mature animal they have been gradually modified into a 

 very long and extremely complex suctorial apparatus, but when at 

 a much later time this proboscis became superfluous in a species, 

 because the butterfly or moth, from some cause or another, lost 

 the habit of taking any nourishment at all, its degeneration exercised 

 no efiect on the jaws of the caterpillar, as we can observe in many 

 hawk-moths, silk-moths and Geometridse. How could such a 

 degeneration become transmissible if the caterpillar's jaws, from 

 which those of the adult are developed, remain the same 1 We are 

 thus forced to assume that there is something in the latter which 

 can vary from the germ, without the jaws themselves being altered 

 thereby. This 'something' it is which I call 'determinants,' vital 

 particles, which— however we may try to picture them— are indeed 

 contained in the cells of the caterpillar's jaws, but are there inactive 

 and do not influence the structure of these, while, on the other 

 hand, it is their constitution which determines the form and structure 



