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wetter west, with a colony oi Xylophasia monoglypha on each 

 side of the range. Then let us make use of one of the theories 

 concerning the cause of melanochroism, and suppose that 

 the colony on the east side will remain light in colour and 

 that on the wetter side will become after a time very dark. 

 If we then further imagine that the moths from the light 

 colony by some chance interbred with dark moths from the 

 other colony, we can easily see that an inner tendency to 

 variation would be set up, especially if we remember another 

 much-discussed theory — the law of Mendel. It seems 

 necessary to suppose the existence of an innate tendency to 

 vary when we consider such species as Apamea didyma, 

 Agrotis tritici, Acalla cristana, and Cerostoma radiatella. All 

 these species appear to vary excessively without any regard 

 to local or atmosphere influences. 



The degree of evolution attained by each lepidopterous 

 insect to-day equals the sum of the responses made in every 

 stage by all its ancestors, and by the individual itself, to the 

 demands of those special inner and outside forces which 

 have been working on the race ever since its existence began. 

 Some of these forces we are aware of, but most of them are 

 probably still unknown to us, though it seems perfectly evident 

 that they cannot all of them be the same in each stage of 

 lepidopterous life : and that, therefore, the effects w^hich 

 they produce on the insect in each stage are peculiar to that 

 stage, and need not have any bearing on the evolution of 

 the other stages. We may look on each stage as having its 

 own life, its own functions, its own environment, and its own 

 evolution. 



In some way, however, it appears absolutely necessary to 

 assume that though the evolution of one stage does not 

 interfere with that of another, that though each stage evolves 

 in response to stimuli, some of which work on that stage alone, 

 some alteration of the germ-cells, or those parts of the insect 

 which carry on the hereditary tendencies, must take place. 

 The advance made by each individual in each stage must in 

 some way be stored up and carried over to the next genera- 

 tion, otherwise each individual would have to evolve de 

 novo, and no progress of the race would be made. How or 

 where these arcana are preserved we have no knowledge ; 

 but when we have before us the minute spherical egg of 

 Hepiahis luptdinus, or the beautifully sculptured egg of Pyra- 

 meis cardui, with its miniature Roman aqueducts guarding 

 the micropyle, or even that microscopic speck of life, the egg 

 of Neptictda acetosce, we can, if these eggs be fertile, be sure 



