78 



SCIENCE-GOSSIP. 



marked differences would assist greatly in the 

 rapidity of adaptation. 



Value of Recognition Marks. 



The value of these "recognition marks" is 

 probably of some importance in many species, yet 

 the presence of such marks does not prevent the 

 crossing of such species as Smerin tints ocellatus and 

 S. populi, of Amphidasys strataria and A. betularia, of 

 Ennomos quercinavia (angularia) and E. autumnaria, 

 when opportunity offers. Still, there can be no 

 doubt that, in Lepidoptera. specialization of andro- 

 conial scales, scent tufts, and similar structures 

 may have much to do with the usual natural 

 mating of species, especially in view of our present 

 knowledge of the sight of insects, and the doubt 

 that may naturally arise as to their want of ability 

 to discriminate small and trifling colour marks, 

 such as those which usually exist between two very 

 closely allied species. Nor is the colour sense of 

 insects sufficiently defined to prevent the pairing of 

 a perfectly normal specimen of A . betularia with a 

 perfectly melanic aberration of the species, nor 

 the correct pairing of the various forms of a poly- 

 morphic species like Cidaria immanata. It would 

 appear certain, therefore, that the correct pairing 

 of species in nature among Lepidoptera is often due 

 to causes other than recognition marks, however 

 important a part they may play in certain cases. 



Isolation. 



Although I do not see that mutual infertility 

 would be, as Wallace asserts, brought about by 

 natural selection wherever two incipient species 

 were in contact, yet it is quite clear that, in some 

 way, nature must prevent their crossing, if a new 

 form, or forms, is to be differentiated. Now, it is 

 quite clear that, to prevent this, isolation of some 

 kind must occur. After a careful consideration of 

 the matter, it appears to me that, among Lepidop- 

 tera at least, the isolation is frequently more or 

 less perfectly brought about by a difference in the 

 time of year at which the imago reaches the 

 perfect stage. Among our butterflies the single 

 brood of Limenitis Sibylla falls between the two 

 broods of the allied L. Camilla, and, in this case, 

 there is not even a differentiation of the food- 

 plants of the two species, both being confined to 

 honeysuckle. Similarly, the single-brooded Poly- 

 ommatus corydon falls between the two broods of 

 P. bellargus, nor must it be thought that these 

 species are so distinct as the colour of their res- 

 pective males would suggest, for Buckler and 

 Hellins were both unable to definitely distinguish 

 the larvae, whilst I have in my possession an 

 undoubted wild hybrid of these two species. I 

 need only call attention to the single-brooded 

 Cidaria immanata, which occurs in July and 

 August, at a time practically intermediate between 



the two broods of C. russata ; to the parallel 

 instance of the single-brood of Tephrosia crepuscu- 

 laria, which appears between the two broods of 

 T bistortata. True, the emergences may occasion- 

 ally overlap for a few days, locally ; but for all 

 intents and purposes the species are as perfectly 

 isolated as if their habitats were separated by 

 some of the most marked physical barriers. 

 Isolation, indeed, must be the essential factor of 

 the differentiation of new species, and an isolation 

 that is engendered by the physical inability of two 

 species to appear in the imaginal state at the same 

 time is as potent as any physical barrier that 

 prevents the two incipient species spreading to 

 each other's grounds. It is only isolation of the 

 kind pointed out above that could allow of the 

 formation of a new species under the same general 

 conditions and on the same ground as that 

 occupied by its immediate progenitor. Given this 

 isolation, the close interbreeding of the individuals 

 of the new form and the intensification of its 

 peculiarities are as assured as is the powerlessness 

 of the new to cross with the ancestral form. The 

 formation of peculiar androconia and other charac- 

 ters will also tend to specialize the new form, and 

 when once the peculiarities of the new form, albeit 

 at first modifications of the old form, have become 

 fixed, the possibility of crossing frequently in 

 nature, even if subsequent changes lead to their 

 appearance at the same time, will be much lessened 

 thereby. How strong a factor this may really be 

 in the differentiation of closely allied species will 

 be evident to every field entomologist. Brenthis 

 euphrosyne and B. selene offer an illustration. In 

 dozens of closely allied Noctuid moths its influence 

 is evident, e.g.,Agrotis trilici and A . obelisca, occurring 

 in July-August and late September respectively. 

 Caradrina ambigua and C. taraxaci, Agrotis segetum and 

 A. lunigera, Triphaena subsequa and T. orbona (comes), 

 etc. In none of these are there any real " recog- 

 nition marks " in the true sense of the word. 

 (To be continued.) 



There is a persistent rumour that Sir William 

 Crooke's presidential address this year at Bristol, 

 to the British Association, is to cause some sensa- 

 tion and be a of startling character. 



Structure of Butterflies' Wings.— Mr. A. 

 Radcliffe has made a comparative study of the 

 venation of butterflies' wings. He sums up his 

 results in the " Transactions of the Entomological 

 Society of London," wherein he states that the 

 changes in the veins " take a direction which 

 stands probably in relation to the mode of flight." 



Conference of Astronomers. — Last year's 

 Conference at the Yerkes Observatory having 

 proved so successful, it is proposed to make the 

 gathering annual. The "American Association 

 for the Advancement of Science " will be meeting 

 at Boston on August 22nd, the fiftieth anniversary 

 of its foundation, so the conference will be held at 

 Harvard College Observatory, August 18th, 19th 

 and 20th. 



