42 



SCIENCE-GOSSIP. 



lime which they find to serve in the material for 

 their shells. The large Roman snail {Helix pomatia), 

 is only found in Britain on chalk and limestone 

 hills, as is also the case with Cyclostoma elcgans, a 

 species with an elegant spiral shell, the mouth of 

 which is closed with an operculum, or trap door. 

 It is the only British representative species of a 

 large tropical genus. The trunks of beech trees 

 on a chalky soil are the favourite resort of various 

 small snails of the genera Biilimus, Clausilia, etc. ; 

 while dry stony places are frequented by minute 

 kinds, such as Pupa and the small species of Helix. 

 The rain which falls on a chalky or limestone 

 soil in great part sinks into the ground and reissues 

 at the lowest point at which it can find an exit in 

 copious springs of clear water, forming streams 



which maintain their volume and temperature 

 comparatively constant throughout the year. 

 Hence in cold weather these streams are warmer 

 than the air, and the water-weeds, such as various 

 forms of Potamogeton, Ramcnculus, Ceratophyllum, 

 etc., with which they are often full, maintain 

 their growth through the winter. The clear chalk 

 streams are also frequented by trout and other 

 freshwater fish, such as miller's thumb, loach, 

 bleak and grayling. The freshwater crawfish is 

 also only found in hard-water streams, but prefers 

 those which are somewhat muddy. Freshwater 

 molluscs are also plentiful in such streams, for 

 the same reason that the land species are on 

 a chalky soil. 



{To be continued.) 



ORIGIN OF SPECIES IN INSECTS.^ 

 By J. W. TuTT, F.E.S. 



Doubtful Species. 

 VTEAR by year we are faced, in some form or 

 other, with the question : " What is a 

 species ? " Year by year this question is discussed 

 in our magazines without getting any nearer to 

 a definite issue ; year by year the discussion will 

 go on so long as there are those who think that 

 every species can be defined with unerring accuracy, 

 so long as there are those who think that every 

 species is distinctly cut off from its nearest allies, 

 that there are no species in the process of making, 

 as it were. We have for several years discussed 

 the specific identity, and the reverse, of Tephrosia 

 crepusciilaria and Tephrosia bistortata, and have 

 arrived at no very satisfactory results. Everyone 

 seems willing to grant that they are what may 

 be called " doubtful " species, that is, that the 

 characters by which they may be differentiated 

 from each other are not sufficiently decided to 

 leave no doubt, in some cases, as to which species 

 a particular individual specimen should be 

 referred. In other words, it is stated, that even 

 specialists cannot invariably determine them. 



• Hybridity of Allied Species. 

 The practical work which Mr. A. Bacot and 

 Dr. W. S. Riding, F.E.S.,have successfully carried 

 out in the direction of hybridising these species, 

 appears to me to be of the greatest importance, 

 and the results arrived at may possibly be very far- 

 reaching in their character. That these species 

 are very closely allied, everyone allows ; that the 

 distinctive characters are so ill-defined as to make 

 it a matter of difficulty, except for the trained 



1 Being part cf a Presidential Address delivered before 

 the members of the City of London Entomological and 

 Natural History Society. 



specialist, to discriminate, may also be conceded. 

 They are species in the making, as I have just 

 said, and their specialisation is not yet completed. 

 It is clear that, if the theory of evolution by 

 natural selection be sound, there must be many 

 cases, and there must be, in nature, every gradation 

 between the polymorphic and unstable species 

 in which almost every individual varies from 

 almost every other in some slight and unimportant 

 manner, through every gradation of varieties (local 

 races) and sub-species to clearly-defined species. 



Each species is separable from its nearest allies 

 by certain characters which will differentiate it 

 from all other species. These we call specific 

 characters. Some naturalists, and I disagree 

 entirely with them, go so far as to include all 

 species that will pair and produce offspring as 

 being one species. Under these conditions we 

 should have to unite Smerinihiis popicli with S. 

 ocellatus, Amphidasys strataria with A. hetulana, 

 Satumia carpini with S. pyri, and a large number 

 of species well-defined on many characters in all 

 their stages of existence. We should also have to 

 unite the pheasants, Phasianus colckicus with P. 

 torquatiis : the hase, Lepiis euvopaeus {timidiis), with 

 the rabbit, L. ciiniculus; and endless other birds and 

 mammals which are abundantly distinct. When 

 the question of hybridity was first studied it was 

 laid down as an axiom that all hybrids were sterile, 

 and when it was found that fertilization between 

 two plants or animals was possible and that fertile 

 progeny resulted, the plants and animals were 

 reduced at once to the rank of varieties. As, 

 however, our data on this subject accumulates, 

 it appears to be certain that a very large number 

 of closely allied, but, in the generally accepted 

 sense of the term, perfectly distinct, species are 



