Darwin- Wallace Celebration, 93 



wedges touching each other and driven inwards by incessant 

 blows. Fully to realize these views much reflection is 

 requisite. Malthus on man should be studied ; and all such 

 cases as those of the mice in La Plata, of the cattle and horses 

 when first turned out in South America, of the birds by our 

 calculation, &c., should be well considered. Reflect on the 

 enormous multiplying power inherent and annually in action 

 in all animals ; reflect on the countless seeds scattered by a 

 hundred ingenious contrivances, year after year, over the 

 whole face of the land ; and yet we have every reason to 

 suppose that the average percentage of each of the inhabitants 

 of a country usually remains constant. Finally, let it be borne 

 in mind that this average number of individuals (the external 

 conditions remaining the same) in each country is kept up by 

 recurrent struggles against other species or against external 

 nature (as on the borders of the Arctic regions, where the 

 cold checks life), and that ordinarily each individual of every 

 species holds its place, either by its own struggle and 

 capacity of acquiring nourishment in some period of its life, 

 from the egg upwards ; or by the struggle of its parents (in 

 short-lived organisms, when the main check occurs at longer 

 intervals) with other individuals of the same or different 

 species. 



But let the external conditions of a country alter. If in a 

 .small degree, the relative proportions of the inhabitants will 

 in most cases simply be slightly changed ; but let the number 

 of inhabitants be small, as on an island, and free access to it 

 from other countries be circumscribed, and let the change of 

 -conditions continue progressing (forming new stations), in 

 .such a case the original inhabitants must cease to be as 

 perfectly adapted to the changed conditions as they were 

 originally. It has been shown in a former part of this work, 

 that such changes of external conditions would, from their 

 -acting on the reproductive system, probably cause the organi- 

 zation of those beings which were most affected to become, as 

 under domestication, plastic. Now, can it be doubted, from 

 the struggle each individual has to obtain subsistence, that 

 any minute variation in structure, habits, or instincts, adapting 

 that individual better to the new conditions, would tell upon 

 its vigour and health ? In the struggle it would have a better 

 chance of surviving ; and those of its offspring which inherited 

 the variation, be it ever so slight, would also have a better 

 chance. Yearly more are bred than can survive ; the smallest 

 .grain in the balance, in the long run, must tell on which 

 death shall fall, and which shall survive. Let this work of 



