VARIATION AND LOCALITY n 9 



or special species come from special places and from nowhere 

 else. In one remarkable case the season of appearance plainly 

 acts as the isolating barrier. Tephrosia bistortata is a small 

 Geometric! moth which has two broods, appearing in March 

 and July respectively. It is closely allied to T. crepuscularia 

 which emerges in May and June. From the fact that occasional 

 specimens cannot be quite certainly referred to one or other of 

 the two, many have held that the two are one species. Never- 

 theless, in general they present distinctions which are plain 

 enough. Some localities have one form only, but in several 

 woods they co-exist. Experiment has shown that the two can 

 be crossed, and that the cross-breds can breed inter se and with 

 at least one of the parent stocks. 1 Some diminution in fertility 

 was observed, but perhaps not more than is commonly encountered 

 when wild forms are bred in captivity. In such a case it can 

 scarcely be doubted that the distinctness of the two forms in 

 the places where they co-exist is maintained by the seasonal 

 isolation. 



Just as the consequences of isolation are to be seen in the 

 most different forms of life so may they also affect the most di- 

 verse features of organisation, such as size, colour, sculpture, 

 shape, or number of parts. In the Sloth (Choloepus) the geo- 

 graphical races differ in the number of cervical vertebrae — or 

 in other words, in the distribution of vertebral differentiation. 

 The geographical races of Cistudo differ in the number of claws 

 and phalanges. 2 



In Shetland, the males of Hepialus humuli (the Ghost Moth) 

 are not sharply differentiated in colour from the females, as they 

 are elsewhere, but in varying degrees resemble them. 3 No such 

 males are found in other localities, and even in the other Scottish 

 islands they are normal. In the island of Waigiu the converse 

 phenomenon has been observed in Phalanger maculatus. Gen- 



l Fov the evidence see Tutt, J. W., Trans. Ent. Soc, 1898, p. 17. Compare 

 the remarkable case given by Gulick {Evolution Racial and Habitudinal, p. 123) 

 of the two races of Cicada, which are separated by reason of their life-cycles, one 

 having a period of 13, the other 17 years. 



2 For references see Materials, p. 396, and also G. Baur, Amer. Nat., 1893, 

 July, p. 677. 



3 Jenner Weir, Entomologist, 1880, XIII, p. 251. 



