D.— ZOOLOGY 95 



a semi-erect position in order to keep its balance. As far back as 1885 

 Leuthner * showed that in the genus Odontolabis the size of the mandibles 

 of the males increased with the size of the body, but that there was never- 

 theless a certain amount of dimorphism, some large males having short 

 mandibles, in one species the largest measured male having shorter 

 mandibles than a smaller male. Similarly, it has recently been shown by 

 Arrow ^ that in Onthophagus, a genus of Dung-beetles, the horns of the males 

 conform in general with the above law, but that in one instance there are 

 long-horned males and short-horned ones of the same body-size. Such 

 exceptions from general rules are of great interest, and it is therefore 

 the duty of the systematist who comes across an exception — generally 

 accidentally — fully to record it. Does it not seem evident from the cases 

 mentioned that Nature can break a rule of development, just as Nature 

 has created species and destroyed them ? After all, the law is only our 

 deduction based on the organisms we find provided by working methods 

 of Nature we endeavour to discover. Circumstances may arise which 

 interfere with the usual ' routine ' of growth. In Papilio meninon L., for 

 example, one of the many Swallowtail butterflies with a polymorphic 

 female, one of the female forms has a spatulated tail and therefore a larger 

 wing-surface than the other females, but its body is not larger than in 

 the specimens which have no appendage. The species is derived from 

 an ancestor in which all specimens had tails. The direction of develop- 

 ment in this and other species is towards taillessness ; but mimetism 

 stepped in and preserved the tail by modifying the course of evolution. 

 The rule of growth illustrated by the Stag-beetles, and corroborated by 

 breeding of plants and animals, leaves no doubt that the characteristics in 

 size and weight of an individual are not inherited and therefore are of no 

 importance in the evolution of species. The test can be made in collections 

 by comparing the closely related species of a genus with each other. In 

 Xenocerus, for instance, a genus of Anthribid beetles, of which we happen 

 to have the largest collection at Tring, the largest specimen [1^) of the 

 largest species has the antenna two and a half times as long as the body, 

 while in several smaller species the antenna is five times as long as the body 

 in the largest male. If the proportional size of body and antenna were 

 constitutional, the largest species in a group of nearly related species 

 should always have the longest antenna, which is not the case. An 

 interesting contradiction of another type in the evolution of allied species 

 has lately come to our knowledge while I was arranging the American 

 Syntotnidee and Arctiidce, families which are among my favourite groups 

 of Lepidoptera. The families are separated in Hampson's classification 

 by vein 8 of the hindwing being present in the Arctiidce and absent in the 

 Syntomidee. Variation in the state of development of this vein, therefore, 

 is of some importance in the systematics of these families. Now, in the 

 genus Neidalia one species in my collection has vein 8 represented by 

 a distinct spur in the ^, whereas in the $ it is a fully developed vein ; 

 in a second species the vein has disappeared in the ^, but remains un- 

 reduced in the ^. That is to say, in the evolution of the neuration of the 



* Trans. Zool. Soc. Lond., vol. xi, p. 385 (1885). 

 ' Trans. Ent. Soc. Lond., 1928, p. 76. 



