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♦ KNOWLEDGE 



[March 1, 188/ 



doubled in twenty-five years, and at this rate in less than 

 a thousand years there would literally not be standing room 

 for his progeny."* If all the offspring of the elephant, the 

 slowest breeder known, survived, there would be in 750 

 years nearly nineteen million elephants alive, descended 

 from the first pair. If the eight or nine million eggs which 

 the roe of a cod is said to contain developed into adult cod- 

 fishes, the sea would ciuickly become a solid mass of them. 

 So prolific is its progeny after progeny, that the common 

 house-fly is computed to produce twenty-one millions in a 

 season ; T\-hile so enormous is the laying power of the aphis, 

 or plant-louse, that the tenth brood of one parent, without 

 adding the products of all the generations which precede 

 the tenth, would contain more ponderable matter than all 

 the population of China, estimating tliis at five hundred 

 millions ! 



It is the same with plants. If an annual plant produced 

 only two seeds yearly, and all the seedlings survived and 

 reproduced in like number, one million plants would be 

 ])roduced in twenty years from tlie single ancestor. Should 

 tlie increase be at the rate of fifty seeds yearly, the result, if 

 unchecked, would l)e to cover the whole globe in nine years, 

 leaving no room for other plants. The lower organisms 

 multiply with astonishing rapidity, some minute fungi in- 

 creasing a billionfold in a few hours, and the protococcus, 

 or red snow, multiplying so fast as to tinge many acres of 

 snow with its crimson in a night. But we need not give 

 further examples of this fecundity whereby Natiu'e, " so 

 careless of the single life," secures the race against extinction. 

 V. The result is obvious : a ceaseless strugijle for food 

 and place. — In that struggle the race is to the swift and 

 the battle to the strong ; the weaker, be it in brain or body, 

 going to the wall, the vast majority never reaching maturity, 

 or if they do arrive at it, only to be starved or slain. As 

 amongst men, competition is sharper between those of the 

 same trade, so throughout the organic ^vorld the struggle is 

 less severe between difterent species than between members 

 of the same species, because these compete most fiercely for 

 their common needs — plants for the same soil, carnivora 

 for the same prey. But whether the battle is fought between 

 allied or unallied sjjecies, the victory is never doubtful — it is 

 assured to the plant or animal that has some advantage, 

 however slight, which its opponent lacks. Among plants 

 growing in a dry soil, those whose leaves have thicker haii'S 

 upon them will absorb more moisture from the air than 

 plants with less hairy leaves, and competing successfully 

 with these, will survive to transmit their advantageous 

 variations. Again, such as are better able to resist the 

 depredations of burglarious insects by protection of thorny or 

 prickly stems, or by nauseous taste, will thrive and multiply, 

 while plants lacking these defences dwindle and become 

 extinct. So with those which by showy colours of their 

 flowers and sweeter nectar attract insects whose visits are 

 desired as carriers of pollen from stamens to pistils. These 

 secure propagation, while plants less attractive remain 

 barren. The birds that are strongest on the wing reach the 

 land whither they migrate, while the weaker perish by the 

 ■way. The lions of sharper sight and more supple spring, 

 the wolves of keener scent, secure their prey, while the 

 feebler members starve. It is with man as with the 

 organisms below him : the quickest in intellect, and those 

 with greater ])Ower of endurance, distance the weak or the 

 .stupid, who fall behind, and, finally, slip out of the ranks 

 altogether. 



But the subtlety and variety of the conditions upon 

 which natural selection seizes escape the keenest observers. 

 Of their success, however, in tracing the varying fortunes of 



♦ " Origin of Siiecies," sixth ed , p. 51. 



species Darwin gives a striking illustration in his explana- 

 tion of the absence of wild oxen and horses in Paraguay. 

 This is due to the action of a small fly which lays its eggs 

 in the navel of newly-born calves and foals, the maggots 

 hatched from the eggs causing the death of the young 

 animals. Now, supposing this parasitic fly to be destroyed 

 bv an insect-eating bird, oxen and horses would abound in a 

 wild state, and as they would eat certain plants, the vegeta- 

 tion would be altered, and these changes in the flora and 

 fauna would involve changes of increasing complexity. 



The inter-relation between the proportion of old maids 

 and an abundance of led clover is not, primd facie, quite as 

 obvious. But it may be proved in this wise. The clover 

 is fertilised by humble-bees, the number of which is deter- 

 mined by the number of field-mice which destroy their 

 nests. The number of field-mice, again, is determined 

 by the number of cats, and the number of these finally 

 by the number of old maids who keep them ! Therefore, 

 as I'ed cloi'er is excellent food for cattle, and cattle are 

 excellent food for man, elderly spinsters are benefactors 

 to their species ! 



The important part played by colour and mimicry in the 

 struggle for life has been demonstrated by Darwin, Wallace, 

 Bates, and other acute observers. The more closely that an 

 animal approximates in form and hue to its surroundings, 

 the easier does it escape detection by its puisuer, and the 

 easier does it avoid the notice of the prey which it pursues. 

 In conformity with this we find that most animals are 

 protectively coloured, while those which are not are so 

 constituted as to render such protection needless. As 

 illustrative of the operation of natural selection in this 

 matter, we may borrow an admirable example from Mr. 

 Grant Allen.* 



la the desert, with its monotonous sandy colouring, a black insect 

 or a white in.sect, still more a red insect or a blue insect, would be 

 immediately detected and devoured by its natural enemies, ttie birds 

 and tbe lizards. But any greyish or yellowish insects would be less 

 likely to attract attention at tirst siuht, and would be overlooked as 

 long as there were any more conspicuous individuals of iheir own 

 kind about for the birds and lizards to feed on. Hence, in a 

 very short time the desert would be depopulated of all but the 

 greyest and yellowest insects ; and among these the birds would 

 pick out those which differed most markedl,v in hue and shade from 

 the sanil around tliem. But those which happened to vary most in 

 the direction of a sandy or spotty colour would be mo-t likely to 

 survive, and to become the parents of future generations. Q'hus, 

 in the course of long ages, all the insects which inhabit deserts liave 

 become sand-coloured, because the least sandy were perpetually 

 ]iicked out for destruction by their ever-watchful foes, while the 

 most sandy escaped, and multiplied and replenished the earth with 

 their own likes. 



Thus, then, is explained the tawny colour of the larger 

 animals that inhabit the desert ; the stripes upon the tiger 

 which, parallel with the vertical stems of liamboo, conceal 

 him as he stealthily nears his prey, the brilliant green of 

 tropical birds, the leaf-like form and colours of certain 

 butterflies, the dtied twig-like form of many caterpillars, 

 the bark-like appearance of tree-frogs, the harmony of the 

 ptarmigan's summer plumage with the lichen-coloured stones 

 on which it sits, the dusky colour of creatures that haunt 

 the night, the bluish transparency of animals which live on 

 the surface of the sea, the gravel-like colour of flat fish that 

 live at the bottom, and the gorgeous tints of those that swim 

 among the coral reefs. 



Among tbe secondary causes of modiScation of species 

 Darwin gives prominence to " sexual selection," or the 

 struggle between males for the possession of females, the 

 result being that the stronger males secure mates and trans- 

 mit the qualities which have given them the mastery to their 



* " Charles Darwin," p. 07. 



