OCR Text |
Show 106 NATURAL SELECTION. [CRAP. IV. one species of grass would always have the best ranc; of succeeding and of increasing in numbers, a;n~ t u~ 0 supplanting the less distinct varieties; and vanet1es, 'W~lef rendered very distinct from each other, take the ran o species. f The truth of the principle, that the greatest amount o life can be supported by great d.iversification of structur~, is seen under many natural cucuinstances. I'?- all: extremely small area, especially if freelJ ov~n to IIDID~gr~tion, and where the contest betweel_!- Individual. and.In~lvidual must be severe, we always find great ~Iversity In its inhabitants. For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, sul?ported twenty species of plants, and these belonsed to mghteen genera and to eight orders, which s~o~s h~w much these plants differ from each othe.r. S~ It IS With th~ plants and insects on small and uniform Islets ; and so In sm~ll ponds of fresh water. Farmers find that they can ra1se most food by a rotation of plants belonging to the most different orders : nature follows what may be called a simultaneous rotation. Most of the animals and plants which live close round any small.piece of ground, .cou~d live on it (supposing it not to be In any way pecuhar In its nature), and may be said to be striving to the utn;ost to live there; but, it is seen, that where they come Into the closest competition with each other, the advanta~es of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shaH, as a general rule, belong to what we call different genera and orders. The same principle is seen in the naturalisation of plants through man's agency in foreign lands. It might have been expected that the plants which have succeeded in becoming naturalised in any land would generally have been closely allied to the indigenes ; for these are con;~ monly looked at as specially created and ada1)ted for then own country. It might, also, perhaps have been expected that naturalised plants would have belonged to a few CHAP. IV. DIVERGENCE OF CHARACTER. 107 groups more especially adapted to certain stations in their new homes. But the case is very different· and Alph. De Candolle has well r.emarked in his great 'and admirabl. e work, that floras e;ain by n~turalisation, proportionally with ~he number of the. native ge"?-era and species, far ~ore In n~w genera than In new species. To give a single Instance : 1n the last edition of Dr. Asa Gray's ' Manual ?f the Flora of the Northern United States,' 260 naturalIsed plants are enumerated, and. these belong to 162 genera. ~ e t~us see that these natr~rahsed plants are of a highly diversified nature. They differ moreover to a large extent from the indigenes; for out of the 162 genera no less than 100 genera are not there indigenous and' thus a large proportional addition is made to the g~nera of these States. .By considering the nature of the plants or animals wh1ch have struggled successfully with the indigenes of any country, and have there become naturalised we can gain some crude idea in what manner some of th~ natives would have had to be modified, in order to have gained an. advantage over .the other ;uath:e.s; ~nd we may, I tlnnk, at least safely Infer that diversification of structure amounting to new generic differences,. would have bee~ pro£. table to them. The advantage of diversification in the inhabitants of the ~arne r~~i?n is, in fact, the same as that of the physi? lo~~al diVISI?n of labour in the organs 0f the same 1ndrvidual bodies-a subject so well elucidated by Milne Edwards. No physiologist doubts that a stomach by being adapted to digest vegetable matter al0ne or flesh ~lone, draws most nutriment from these substa~ces. So In the general ~conomy of any land, the more widely and per~ectly t~e anima~s and plants are diversified for different habits of hfe, so Will a greater number of individuals be capable. of there supporting themselves. A set of animals, With their orgarnsation but little diversified could hardly compete with a set more perfectly diversifled in structure. It may be doubted for instance whether the .£\ust~alian m~rsupials, whicJI are divided into groups dlffenng but httle from each other, and feebly represent- |