Nov. 1, 1S68.] 



HARDWICKE'S SCIENCE-GOSSIP. 



243 



distribution is, far more often than we are generally 

 inclined to suppose, the chief pretection of many a 

 species. 



In islands the various species have usually fewer 

 competitors, and therefore become modified with 

 reference to their competitors alone. Consequently, 

 in islands we find most of the species differing from 

 those of the mainland, more or less, according to 

 the length of time the island has been isolated. 

 (Isolation, acting as a bar to migration, may be men- 

 tioned as the chief reason why islands are usually 

 poorer in species than continents.) If a continental 

 species should be introduced! into an island which 

 has been isolated sufficiently long for its productions 

 to have assumed an insular character, from the larger 

 number of species with which it has had to contend, 

 it will have a great advantage over the indigenous 

 species, and will probably soon become common 

 throughout the island, while the insular species 

 which come into competition with it will run great 

 danger of utter extermination. It also happens 

 that when species are transplanted from one con- 

 tinent to another, they sometimes find their new 

 circumstances so favourable that they are enabled 

 to increase to a great extent, as the horse has done 

 in America. 



When a species once loses ground, it is rarely 

 able to recover it; its enemies press it harder, it 

 becomes rare, it is weakened by interbreeding, and 

 interbreeding itself is unfavourable to variation ; 

 and from all these causes favourable circumstances 

 or variations present themselves less and less often 

 as the species becomes more reduced in numbers ; 

 and finally it sinks from the surface of the great 

 ocean of life, and no trace, except, perhaps, a few 

 chance fossils, remains to showthatit has ever existed. 



Sometimes a species will in the course of ages 

 change its babits, in which case it may happen that 

 organs of great former importance become useless 

 to it under its new circumstances. Sometimes these 

 organs are retained, as in the case of certain species 

 of geese which are web-footed, but seldom or never 

 swim ; but in other cases, as in the wingless beetles 

 of Madeira, and in the blind or semi-blind animals of 

 great caves, disused organs become more or less 

 atrophied, but never so much as to show no trace 

 of their former existence. 



It is commonly asserted that hybrids, when pro- 

 ducible at all, are infertile, while mongrels are 

 always fertile. But Mr. Darwin has shown that 

 these statements are only partially true, and that 

 though domestication often tends in the first instance 

 to produce sterility in individuals of the same 

 species, yet it afterwards facilitates the inter- 

 breeding of distinct species.' To take an extreme 

 instance, no difficulty is experienced in inducing any 

 of our races of domestic cattle to interbreed, yet 

 these are well known to have descended from at 

 least three, if not more, distinct wild species. 



It is often objected that geology furnishes us with 

 no direct proofs of the progression of species ; but 

 this objection would only be valid if our palseonto- 

 logical collections could be assumed to be tolerably 

 complete. But years in geology are as miles in 

 astronomy ; and who need wonder, when such im- 

 mense numbers of living species are daily discovered, 

 even in well-explored countries, that our collections 

 should contain not the ten-thousandth part of the 

 innumerable distinct species (to say nothing of all 

 the intermediate varieties, as is sometimes so un- 

 fairly demanded) that have lived and died on the 

 earth since it was a ball of incandescent lava? 

 Again, millions of years are often represented in 

 our cabinets by a very few chance remains of a very 

 few species ; and to claim that the geological record 

 should be perfect in any sense would be absurd. 



But it may be asked, How is it that, when large 

 animals become extinct, their pigmy representatives 

 often remain ? Are the sloths of South America, 

 which are no larger than monkeys, to be considered 

 as the improved descendants of the gigantic me- 

 gatherium, scarcely inferior to an elephant in size ? 

 The absurdity is obvious, and the difficulty is real, 

 and yet it apparently admits of a very simple ex- 

 planation. When a genus or family is dominant in 

 any country, species of all sizes exist together, as 

 we see with the kangaroos of Australia, which vary 

 from the size of a rat to that of a large sheep, 

 according to the species. But when a group or 

 genus is becoming extinct, the hostile influences 

 will act first and most forcibly on the larger species, 

 which must always be fewer in number and less 

 able to u take advantage of accidentally favourable 

 circumstances than the smaller ones. It is thus 

 reasonable to conclude that when a group or genus 

 has lost its ascendancy, the larger species will gene- 

 rally be the first to disappear, while some of the 

 smaller may still continue to exist for many cen- 

 turies after their former companions have become 

 extinct. 



The facts of geographical distribution may be con- 

 sidered to establish the axiom, almost universally 

 admitted, that every species has been created or 

 developed in one spot of the earth alone, from 

 whence it has gradually spread itself over the dis- 

 trict, large or small, which it now inhabits. We say 

 created or developed to indicate the practical 

 identity of the words as applied to Nature. The 

 terms are often considered antithetical, but there is 

 no real reason why they should be. No one can ex- 

 plain why distinct but closely allied species should 

 be found on a continent and its islands, or hi the 

 Arctic regions and the mountains of temperate 

 climates, on any other theory than that of common 

 descent. Equally difficult, on any other grounds, is 

 it to explain why species, in exactly similar con- 

 ditions, should be confined to different sides of a 

 continuous mountain-chain. It might, indeed, be 



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