300 THE CAUSES WHICH 



we are led to conclude this hannony has been the work of time. 

 With regard to the chemical aspect of the problem, Mr. Meldola 

 has stated that the colouring matters of some plants can be found 

 by the spectroscope in the tissue of caterpillars which feed upon 

 them in an unaltered condition, and the viscera of many is so 

 transparent that the undigested food in the stomach confers on 

 the individual its proper tint, as the yolk often does to an egg. 

 Thus far, then, change in diet produces change in the larva. 

 Passing to the markings, it has been frequently remarked by 

 Sir John Lubbock and some other naturalists, among them Mr. A. 

 G. Butler, that the longitudinal and diagonal lines of caterpillars 

 resemble the veins of grasses and the divaricating ribs in the 

 leaves of trees, enhanced by an illusion of warm shadow- streaks. 

 Dr. Weisman, commenting on this circumstance in the Elephant 

 Hawk Moth, where the young caterpillars retain their longitu- 

 dinal lines only until the third moult, when they are replaced by 

 diagonal ones and eye-spots, infers that the young larva repre- 

 sents an old form, and that the species in the lapse of ages has 

 gone through the stage each individual now completes in a few 

 weeks. This, should the fundamental fact be universal, would 

 harmonise with the geological opinion that the monocotyledons 

 preceded the dicotyledons. The Smaller Elephant, again, leaves 

 the egg with a subdorsal line the larger species does not acquire 

 until the first moult, and is thus possibly a more recent form. 



Having in former chapters traced, however imperfectly, causes 

 which tend to collect and distribute forms of insect life ; having 

 seen how the stimuli of love and rivalry inspire music and prompt 

 dances, and how these conspire, with other agencies such as a 

 common food, to collect certain kinds into flocks, we will now 

 proceed to examine how such swarms are circulated and dissemi- 

 nated by exterior causes^ which, acting mostly in opposition to 

 volition, subject the species to influences operating such organic 

 modification as we have just been noticing. For while on the 

 one hand with insects the tendency to distribution has indirect 

 relation to their locomotive powers, and is greatest in those which 

 are winged, or, again, species are introduced with their food or by 

 animal agency, on the other hand a constant means of dispersion 

 over the earth^s surface must be recognised in the circulation of 

 aerial and aqueous currents, the development and buoyancy of the 



