INTRODUCTION. XIII 



ing most absolutely to their environment. Polia flamcincta, which also 

 rests on lichen covered walls, offers parallel ranges of variation, and 

 whilst the prevailing form of this species at Huddersfield is exces- 

 sively dark with a large percentage of black markings and scarcely a 

 trace of ochreous, those from Driffield in the same county are 

 ochreous in colour, and it is of rare occurrence for a single specimen to 

 be noticed so dark as the Huddersfield specimens. In Somersetshire, 

 a tendency to a reddish tint prevails, whilst some splendid dark 

 specimens, darker perhaps than the Huddersfield specimens, which 

 were captured in Guernsey by Mr. A. J. Hodges, assimilated exactly 

 to the rocks on which they rested. Again, whilst the specimens viEpunda 

 lichenea from Plymouth are dark green in colour, mottled with red, 

 and similar to the Lancashire specimens ; those from Portland (a 

 locality comparatively near to Plymouth), but obtained from the pale 

 limestone rocks, are of a pale silvery-white colour with the red and 

 green reduced to a minimum. Here we see that in distant localities, 

 similar surroundings produce the same or similar phases of variation, 

 whilst, given a different environment, a different phase of variation 

 becomes apparent. The reason of these special forms in special dis- 

 tricts is easily explained. Those specimens which assimilate most to 

 their surroundings, escape their many enemies, and hence, year after 

 year a gradually increasing number of specimens of that particular 

 form, which most readily assimilates to its surroundings, is developed 

 and a local race is formed. Turning from those species that rest nor- 

 mally on lichen-covered rocks and walls, we find among those species 

 which normally rest on fences, trees, &c., that many, such as Cucultia, 

 Xylophasia, Axylia, &c., assimilate exactly, to pieces of cut wood, &c., 

 but the great mass of our NOCTU^B rest on the ground, and hence the 

 geological character of a district has much to do with colour. Agrotis 

 ripce is white, ochreous, reddish, or greyish in ground colour, according 

 to the sand on which it rests. Agrotis valligera, A. cinerea, A. cursoria, 

 A. tritici, A. nigricans, &c., also go through various shades of colour, 

 according to locality, as do various species in other genera. The 

 genera Leucania, Nonagria and their allies offer a good illustration of 

 structural variation responding to environment in another direction. 

 The species of these families sit head downwards, with their wings 

 closely appressed to the reed, &c., on which the particular species feeds, 

 and there look exactly like nodes on the stem. Endless other 

 instances might be cited, but sufficient appear to have been pointed 

 out, bearing on the general subject of " natural selection," especially if 

 we consider that all species exhibit some phase, or are modified by it 

 in a greater or less degree. 



Turning now to the causes which accompany or modify the all- 

 important " natural selection." In all species there appears to be an 

 inherent tendency to vary if placed under distinctly and decidedly ab- 

 normal conditions, whatever such conditions may be. The ordinary 

 normal conditions of a locality tend to produce the normal forms found 

 in that locality, whilst abnormal conditions develop latent tendencies, 

 and variation is the result. It follows that if such abnormal conditions 

 become gradually permanent in any locality and what were previously 

 abnormal conditions become normal, a permanent change will take 

 place in the character of the race inhabiting such a locality. That 



