96 SECTIONAL ADDRESSES 



hindwing from the Arctiid type with 8 fully developed to the Syntomid 

 type with 8 suppressed, the (J Neidalia is in advance of the $. In a second 

 genus, Aclytia, on the other hand, 8 is present in the (J of some species, 

 absent in the ^ of others, with intergradations, and absent in the $ of 

 all species. In this case, therefore, the process of reduction is farther 

 advanced in the $ than in the $, just the opposite of what obtains in 

 Neidalia. The morphology of Aclytia, however, offers an explanation 

 of the contrast. The SS o^ this genus have the costal margin of the 

 hindwing enlarged in conformity with the development of a scent- 

 organ, and vein 8 acts as a support of the lobe. Though the vein is absent 

 from the imago of the Syntomids and ought to be absent from Aclytia, 

 which belongs to the Syntomids, it has reappeared in the cj, being pre- 

 sumably in a recessive or dormant state in the larva and chrysalis, capable 

 of resurrection in the imago if the necessary stimulus arises, which in this 

 case would be the development of a scent-organ. This explanation is 

 perhaps not palatable to those who believe that lost organs are lost for 

 ever ; much depends on what is meant by the word ' lost.' Vein 8 of the 

 Syntomids is probably not really ' lost ' in the individual, but merely 

 suppressed in the imago. 



It must be clearly understood that in speaking of the unimportance for 

 evolution of the bulk of individuals and the size of certain appendages, 

 we referred to specimens of the same country — i.e. individuals belonging 

 to the same interbreeding population. In comparing the populations of 

 two different countries the question assumes quite another aspect. In 

 the systematics of birds the study of subspecies or geographical races has 

 developed into a fine art. Size and shades of colour furnish the main 

 distinctions between subspecies, and here we observe this important 

 contrast that, while the difference of, say, 6 mm. in the wing-lengths of 

 specimens from the same country is of no importance, because not 

 inheritable, the difference of 2 mm. between the populations of two 

 countries is an inheritable quantity and therefore qualifies the two popu- 

 lations as being subspecifically distinct from one another. The evolution 

 of the subspecific size-difference evidently starts with a shifting of the 

 average size. A series of English sparrows has not the same average 

 wing-length as a series from Central Germany (large numbers have actually 

 been measured by Dr. Kleinschmidt) ; in other birds there is only a more 

 or less moderate overlapping in size, and in others again the averages are 

 so far apart that there is a gap between the largest bird from one country 

 and the smallest from another country. The size of birds is remarkably 

 constant as compared with that of Lepidoptera. In these insects size 

 depends to a great extent on a variable outside factor, the supply of luscious 

 food for the caterpillar. In the dry season of the tropics and in the late 

 summer and autumn of the temperate regions food is hard and the resulting 

 butterflies, therefore, as a rule smaller than those resulting from cater- 

 pillars which have fed up in the wet season or in spring and early summer 

 when food was plentiful and soft. Size, therefore, is as a rule of no great 

 weight in the diagnoses of subspecies of butterflies ; but there are such 

 which are definitely smaller or larger than others — a case in point being the 

 races discovered by Wallace on Celebes and characterised by large size 



