October 6, 1892] 



NATURE 



535 



apply this principle he falls into errors which a little 

 consideration would have avoided. Thus, on p. 21, the 

 following sentence occurs :— " Seeing that pigment has 

 been proved in so many cases to be alterable by changes 

 in the food, it is not surprising to find that as a rule the 

 colours of larvae are totally different from those of the 

 adult form ; " implying that the difference of diet accounts 

 for the difference of colour, — a conviction stated even 

 more strongly on the next page. It is quite sufficient 

 answer to this hasty conclusion to point out that the 

 colours of the imago are just as dependent on the larval 

 food as the larval colours themselves, and that they have 

 made their appearance long before the imago has had 

 the opportunity of feeding. Again, in speaking of the 

 " strong superficial likeness " of the Drone-fly {Eristalis 

 tenax) to a bee, the author hints at likeness of food as a 

 possible explanation (p. 232). "It is an interesting fact, 

 in connection with the resemblance between this fly and 

 a hive bee, that it feeds upon pollen and honey. This fact 

 may have some significance in relation to the effects of food 

 upon form and coloration." But the form and coloration 

 of Eristalis depend upon the food absorbed by its " rat- 

 tailed " larva, living in putrid mud, under conditions 

 utterly unlike those of the larval bee. 



Under the consideration of light as a cause of colour 

 an extremely bad piece of reasoning is adopted from 

 Werneburg (p. 62), who argues that light has an import- 

 ant influence on the formation of pigment during the 

 pupal period. By selecting favourable instances and de- 

 scribing them with an enthusiasm which borders on inac- 

 curacy (e.g., speaking of U. sambucaria as " bright yel- 

 low ") and by neglecting all others, he makes it appear 

 that there is something to be said for this view. 



In the section on " Variable Protective Resemblance 

 in Chrysalids," the results of recent work are given very 

 inaccurately ; the golden colour of pupae is explained 

 as due to " thin films of air or some gas," and it is even 

 suggested that " intense light may cause some gas to be 

 given off in greater abundance." But it was shown years 

 ago that the appearance is due to some lowly refractile 

 liquid, and, in fact, alcohol answers the purpose very well 

 indeed. Gases do not appear to have the power of enter- 

 ing the intervals between the cuticular lamelte, perhaps 

 because the latter come together and obliterate the 

 chinks on the evaporation of the fluid. Again, it is stated 

 that " the pupa was also made to assume a light colour 

 upon one half and a dark colour upon the other." As a 

 matter of fact the invariable failure of the pupa to do this 

 formed the basis of some of the principal conclusions 

 reached. It was also surely unnecessary to quote an 

 ignorant assumption of Elmer's on the subject— an 

 assumption which was not even original, and has been 

 disposed of long ago. 



In favour of the effects of climate reliance is placed on 

 Scudder's conclusion that melanism is only found in the 

 butterflies south of New York, albinism only to the north 

 (p. 55). And yet in Europe melanism is especially preva- 

 lent among the northern moths, from which we may in- 

 fer that the American observations, however they are to 

 be explained, are not direct effects of climate. 



He suggests that the blackness of a lizard on one of 

 the Canaries may be due to moisture ; but these islands 

 NO. I 197, VOL. 46] 



are about as dry as small oceanic islands can be. All 

 the lizards seen by the present writer in Teneriffe and 

 Grand Canary, some three or four species, were dark in 

 colour and harmonized with the tint of the dark dry 

 volcanic rocks on which they were seen, and among 

 which they almost invariably escaped when pursued. 



One suggestion is very remarkable. After giving 

 reasons why he does not consider that the resemblance of 

 Volucella to humble-bees, &c., is to be explained as a case 

 of aggressive mimicry, the author suggests (p. 228), " If 

 wasps and bees have the same unintelligible liking for 

 keeping pets that another group of Hymenoptera— the 

 ants — have, the whole series of facts may prove to have 

 a very different meaning, but one which is not quite in 

 accord with the theory of mimicry on the part of the 

 Volucella." The keeping of pets by ants is so very far 

 from being unintelligible in some of the most important 

 cases {Aphides, Lycaenid larvae, &c.) that we may fairly 

 expect an explanation in other instances. But even if 

 the author's suggestion were valid it would still fail to 

 account for the very point at issue— the great superficial 

 resemblance of Volucella to Hymenoptera. 



On p. 92 he is quite prepared entirely to dispose of all 

 advantages in the struggle for existence in favour of fer- 

 tility ; this alone is enough to prevent extermination. 

 Speaking of the wonderful disguise of Geometer larvae (and 

 if this be not the result of selection it must be admitted 

 that the principle fails indeed) he says, " In the meantime 

 the excessive fertility of the parent moths appears to be 

 a sufficient guarantee against extinction, apart from any 

 subsidiary advantage to be gained by colour protection." 

 It is sufficient reply to this statement to point out that the 

 fertility of these small-bodied moths is very far from ex- 

 cessive when judged from an insect standard ; that if the 

 larvae are offered to any insect-eating animals they are 

 when detected, devoured with the greatest avidity, but that 

 if offered motionless on their natural food-plant they are 

 often passed over ; that insect-eating animals, especially 

 when r earing their young, are by no means fed to reple- 

 tion, so as deliberately to refuse the food they evidently 

 relish. 



It is very confusing after this candid avowal to read a 

 few pages further on (p. 97), " On the whole, it seems 

 more profitable to a caterpillar to adopt protective resem- 

 blance to its surroundings as a means of escaping its 

 foes ; at any rate, this is what actually occurs. ' The main 

 purpose in life of a caterpillar,' says Mr. Scudder, ' next 

 to feeding, is not to be seen.' " 



Many quite irresponsible suggestions, which it would 

 have been wiser to have withheld unless accompanied by 

 at least some evidence, are made or adopted from other 

 writers. Of this nature are the remarks of Leydig on the 

 colours of Helix nemoralis, and the author's suggestion 

 that the dark variety of the female Silver- washed Fri- 

 tillary may be due to the moisture of wooded districts. 



Of some of the author's suggestions we may use his own 

 words, and say, "This explanation has an air of reason- 

 ableness, which might lead to the inference that it 

 had been amply tested by actual experiment " (p. 64). 

 Others however, including some which have been quoted 

 here, certainly appear to lad; this " air of reasonable- 

 ness." 



