Aug. 1, 1867.] 



HARDWICKE'S SCIENCE-GOSSIP. 



173 



leaf, and descends to the surface of the ground, and 

 spins its small silken cocoon, in which it changes 

 to the pupa state ; and iu the following month of 

 June, the little moth emerges from the cocoon, and 

 may be found sitting on the trunks of oak-trees. It 

 is barely a quarter of an inch in the expanse of the 

 wings ; the fore wings are black, with two nearly 

 opposite triangular whitish spots in the middle. 

 This we call Nepticula subbimaculeila (fig. 178). 



In the months of May and June, we of 'en find 

 large brown blotches on the leaves of hawthorn ; 

 and, on holding one of these leaves up to the light, 

 we see that the entire green portion of the leaf has 

 been eaten away, nothing- being left but the upper 

 and lower cuticles of the leaf ; where these brown 



Fig. 179. Mined Hawthorn Leaf, and Larva of Co!eophora 

 nigricella, natural size, and magnified. 



blotches are, moreover, we see in the lower cuticle 

 a small round hole. On some leaves we may find 

 attached a small brown cjdindrical object, about 

 half an inch long ; this is the portable habitation, 

 or case, formed by the larva which has mined the 

 hawthorn leaf, and its mode of proceeding is as 

 follows : it fastens its case to the underside of a 

 leaf, and bites the round hole in the lower skin of 

 the leaf, and then proceeds to devour the fleshy 

 green portion of the leaf. By degrees it eats the 

 green portion of the leaf away all round the spot 

 where its case is fastened, but carefully leaves the 

 skins of the leaf unbroken ; and as it comes 

 further and further out of its case as the mined 

 space becomes larger, and as it has to reach to a 

 great distance for some of the green substance of 

 the leaf, it will frequently happen that it comes 

 entirely out of its case into the leaf; but if in 

 any way alarmed, it retreats quickly to its case, 

 which in due time it transports to another leaf, and 

 repeats a similar process. When full-fed, it gene- 

 rally fastens its case to the upper side of a leaf, and 

 then assumes the pupa state. In two or three 

 weeks the moth makes its appearance, and may be 

 seen sitting on hawthorn leaves with its antennae 



stretched straight out before it ; the expansion of 

 the wings is nearly half an inch, and the forewings 

 are unicolorous — blackish. This we call Coleoplwra 

 nigricella (fig. 179). 



It will thus be seen that the leaf-mining larvae 

 do not all work after the same fashion, but that each 

 sort of larvae has its special work to do, which it 

 does after its kind. We have in this country several 

 hundred leaf-mining larvae of the order Lepidoptera, 

 and possibly, when all the orders are considered 

 which furnish examples of leaf-mining larvae, they 

 may without exaggeration be numbered by thou- 

 sands. 



THE UNITY OF MANKIND. 



TE Mr. Milton had taken the trouble to read my 

 •*- essay carefully before he attempted to criticise 

 it, he would have done himself no harm and me less 

 injustice. He asserts that in my essay I allude to 

 Prichard aud Knox, Pickering and Laurence, as 

 likely to perplex the student instead of aiding him. 

 I say nothing so ridiculous, but simply advise him 

 to avoid those writers who abound in arbitrary and 

 endless classifications. I, of course, do not in the 

 least discourage the study of the religion, language, 

 and customs of every race in order to discover from 

 them its origin and affinities ; but I maintain that 

 while this process is going on, some such memoria 

 technica as this arbitrary division according to 

 colour would afford, is wanted to do what the 

 Linnaean system has done for Botany, i.e., to keep 

 together the various elements of the science until 

 its natural laws and divisions have been ascertained 

 with some degree of accuracy. The division of 

 mankind by colour is, after all, no novelty, but 

 sanctioned by some of the highest authorities 

 extant ; and in proof of this I may refer to that 

 very learned work on " The Geographical Distribu- 

 tion of Mammals," by Mr. Andrew Murray, which 

 has recently appeared, and in which a system very 

 similar to the one which I advised is adopted. Of 

 course Mr. Milton will be able to adduce many 

 exceptions to such a theory. There are doubtless 

 races which cannot be strictly characterised as 

 black, white, or brown ; but those who have studied 

 the subject will, I think, agree with me when 

 I say that, according to our present knowledge, the 

 classification I have adopted is the only one capable 

 of any extended application. 



I do not for a moment believe that heat alone 

 produces a dark skin ; but heat, an unhealthy 

 climate, and prolonged isolation will, I think it is 

 impossible to doubt, produce and perpetuate the 

 most marked and extraordinary peculiarities. The 

 vast desert of the Sahara was, geologists tell us, 

 once the bed of an inland sea which completely 

 severed Africa from the rest of the old world and 



