SCIENCE-GOSSIP. 



43 



not only reciprocally fertile, but their hybrids are 

 also fertile intey se. Before, however, we can assert 

 that we really know anything about the subject, 

 a very great number of careful experiments on 

 many different species must be carried out and the 

 results compared. 



Variations always present. 



The general tendency not only for all organ- 

 isms to vary, but also for every constituent 

 structure and part of an organism to vary, 

 is so well-known, that, in re-reading any of 

 Darwin's works, one is struck by the frequency 

 with which he prefaces his remarks with " if the 

 species vary," or some similar phrase. One has 

 some difficulty in conceiving how vastly our facts 

 relating to variation have increased during the last 

 half-century. As entomologists we know very well 

 that in every individual brood of any given species 

 there is an abundance of variations present, upon 

 which natural selection might work in many direc- 

 tions. If this be once thoroughly understood, and, 

 if to this be added the fact that an inconceivable 

 percentage of the progeny of every living insect 

 (even if only judged by the standard of lack of 

 increase in numbers of common species of Lepi- 

 doptera in well - known localities) is annually 

 destroyed by various causes, nature continually 

 weeding out the less fit, so that only a few selected 

 and well-favoured individuals reach maturity, then 

 one can readily conceive that natural selection may 

 have much to do with the process of the formation 

 of a new race under the most favourable conditions. 

 There can be little doubt that, by a slow process of 

 the selection of suitable variations presented by a 

 species, nursed under the most favourable condi- 

 tions, on lines similar to those already indicated, 

 species have been formed. 



Utility and the Origin of Specific 

 Characters. 



It may not be out of place now to say a few 

 words on the evolution of those " specific charac- 

 ters " which every species possesses, and which 

 ultimately result in the differentiation of each 

 individual species from all others. I have at- 

 tempted to show in certain articles that I pub- 

 lished on "Mimicry" (Entom. Record, vol. viii.) 

 that utility is the mainspring on which the forma- 

 tion of mimetic patterns depends, and that utility 

 has guided natural selection to act in ways advan- 

 tageous to the species in their evolution. I would 

 also urge that utility is again the guide by which 

 natural selection is driven into the paths advan- 

 tageous to the species when it brings about the 

 development of new forms, which ultimately 

 become new species. On this question Huxley 

 says : " Every variety which is selected into a 



species is favoured and preserved in consequence 

 of being, in some one or more respects, better 



adapted to its surroundings than its rivals 



For, as has been pointed out, it is a necessary 

 consequence of the theory of selection that every 

 species must have some one or more structural 

 or functional peculiarities, in virtue of the advan- 

 tage conferred by which it has fought through 

 the crowd of its competitors and achieved a 

 certain duration. In this sense, it is true that 

 every species has been originated by selection." 

 Dr. Alfred Russel Wallace says: "Perhaps no 

 principle has ever been announced so fertile in 

 results as that which Mr. Darwin so earnestly im- 

 presses upon us, and which is, indeed, a necessary 

 deduction from the theory of natural selection, viz., 

 that none of the definite facts of organic nature, 

 no special organ, no characteristic former marking, 

 no peculiarities of instinct or of habit, no relations 

 between species or between groups of species can 

 exist but which must now be, or once have been, use- 

 ful to the individuals or races which possess them." 

 Here it is quite evident that two of the greatest 

 thinkers on this subject accept the principle of 

 the utility of specific characters, at any rate, at 

 the time of their origin as such ; and, although 

 it is possible that certain specific characters may 

 exist in certain species which are now of no direct 

 advantage to their possessor, yet there can be 

 but little doubt that at some previous time in the 

 past history of the species they were either them- 

 selves useful, or were correlated with some useful 

 character. 



Average Characters. 



As I have already pointed out, the amount of 

 variation that occurs in each species year by year 

 is much greater than was originally supposed. Yet 

 the weeding out of the most marked aberrations 

 results in the production of what may be called a 

 general facies, presenting an average of the special 

 characters for each particular species. In spite, 

 therefore, of the variation that exists between the 

 individuals of a species, a similarity is preserved 

 which enables the species to maintain itself in its 

 given environment. 



Individuals of Species not Identical. 



When the materials acted upon are identical, 

 and the conditions under which they are acted 

 upon are identical, we may take it as a general 

 axiom that the same cause will produce similar 

 results. In the true sense of the word, the indi- ■ 

 viduals of no species are identical, and hence the 

 same cause acting upon the individuals under the 

 same conditions does not produce the same results, 

 except in a very general manner. Bearing this in 

 mind, we may proceed to the consideration of a 



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