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HARDWICKE'S SCIENCE-GO SS I P. 



[Nov. 1, 1S68. 



indefinite period to occur in nature. An animal or 

 plant, when domesticated, is inevitably placed in 

 circumstances differing more or less from those under 

 which it has existed in a state of nature. If it is 

 capable of existing in domestication at all, slight 

 modifications which may or may not tend to assimi- 

 late it to its new conditions are nearly sure to occur. 

 If the variation was beneficial to the domesticated 

 species, it would be reproduced, while the original 

 wild form would disappear, in domesticity ; or if it 

 varied so as to become more useful in any way to 

 the owner, it would be preserved, while the in- 

 dividuals which did not vary would be neglected, or 

 perhaps purposely destroyed. A species might vary 

 in this manner in two or more directions ; and when 

 after the lapse of a certain time, the two forms were 

 compared with each other, and with the wild parent, 

 it would very probably appear that there were three 

 distinct species. Still more distinct would the two 

 domesticated forms appear if the wild race had 

 become extinct in the interval. Yet the domesticated 

 forms would still at times display a tendency to 

 revert to the original type, especially if accidentally 

 placed in similar circumstances to those in which 

 the wild species had previously existed. Two 

 remarkable cases of reversion are quoted by Mr. 

 Darwin ; one is that of our domestic pigeons, which 

 "are descended from a pigeon (including two or 

 three sub-species or geographical races), of a bluish 

 colour, with certain bars and other, marks; and 

 when any breed assumes by simple variation a bluish 

 tint, these bars and other marks invariably appear, 

 but without any other change of form or character. 

 When the oldest and truest breeds of various colours 

 are crossed, we see a strong tendency for the blue 

 tint and bars and marks to reappear in the mongrels." 

 Another case, still more remarkable, is that of the 

 various species of horse, (including the ass, zebra, 

 &c.,) all of which can be crossed, and whose hybrids 

 are more or less fertile, although no one doubts that 

 the species are as distinct as in any other mam- 

 malian genus. All these animals have a tendency to 

 become striped, which is most conspicuous in the 

 hybrids ; and even the horse shows this at times, 

 especially when of a dun colour, though the horse is 

 more rarely striped than any other species. Of this 

 Mr. Darwin says, in his graphic language, "Eor 

 myself, I venture confidently to look back thousands 

 on thousands of generations, and I see an animal 

 striped like a zebra, but perhaps otherwise very 

 differently constructed, the common parent of our 

 domesticated horse ; whether or not it be descended 

 from one or more wild stocks, of the ass, the 

 hemiouus, quagga, and zebra." 



In a wild state, species are exposed to fewer 

 influences tending to produce variation; but in 

 every large genus, it is notorious that the dis- 

 tinction drawn between species and varieties 

 is often so slight as to be wholly arbitrary. 



Two- allied species will occur in the same or an 

 adjacent locality, while intermediate forms may occur 

 either with the extremes, or in a different outlying 

 district, the various forms keeping themselves more 

 or less distinct. In the case of a group of large 

 islands, the effect of isolation is often most remark- 

 able. Take for example the case of the genus 

 OrnitJioptera, which contains some of the largest 

 and most beautiful butterflies known. Its extreme 

 limits are India and Australia, but the species arc 

 mostly found in the intermediate archipelago ; and 

 the various islands all possess their peculiar forms, 

 often passing into one another by such slow grada- 

 tions that it is anything but easy to say which forms 

 are entitled to be classed as distinct species, and 

 which should be considered mere varieties. How un- 

 like is the magnificent green of OrnitJioptera Priamus 

 and its allies, to the rich blue of 0. Urvilliana, or 

 the brilliant golden orange of 0. Croesus and 0. 

 Lydius ! The forms differ also greatly in size, shape, 

 and markings ; and yet they pass into one another 

 by such fine gradations that all our best entomolo- 

 gists are now agreed that the twelve or fourteen 

 described species in this group of the genus green, 

 blue, and golden, are merely forms of one variable 

 species. But supposing the intermediate forms were 

 to become extinct, no one would ever dream of class- 

 ing the others together. This instance may serve to 

 show how indefinite is the idea of a species in the 

 minds of even the most eminent naturalists who 

 have not yet learned to regard the terms " species " 

 and " variety " as synonymous. 



It not unfrequently happens that two species will 

 coexist throughout an extensive range, keeping 

 themselves perfectly distinct at one extremity, while 

 at the other they are so blended by intermediate 

 varieties as to be scarcely distinguishable. Thus, 

 two. well-known Alpine butterflies, Pamassius Apollo 

 and P. Delias, keep themselves perfectly distinct in 

 Europe, even the larvae differing ; but in Asia so 

 many intermediate varieties and local sub-species 

 occur, that no satisfactory characters can be certainly 

 relied on to separate them. 



It has already been mentioned that the high geo- 

 metrical ratio of increase of any species imperatively 

 requires an almost equally high counterbalance of 

 destruction to keep it within any reasonable bounds. 

 Mr. Darwin has shown that every animal or plant is 

 dependent for its very existence on an innumerable 

 number of favourable circumstances, and the direct 

 or indirect influence of a great number of other 

 organisms; and that all arc liable to immense de- 

 struction, generally in their young stages. So long 

 as circumstances are sufficiently favourable to allow 

 a species to arrive at maturity in sufficient number? 

 for reproduction, it will hold its ground ; but a very 

 slight balance on the other side will often exter- 

 minate it altogether. A power of adapting itself to 

 circumstances by slight variations over its area of 



