366 JOURNAL OF SCIENCE. 
organic being naturally increases at so high a rate, that, if not 
destroyed, the earth would soon be covered by the progeny of a 
single pair.” And he goes on to give us numerical examples illus- 
trative of this statement. But the fact is one of the commonest 
observation. If we take a square yard of ground in any well kept 
garden,—ground which is supposed to have been kept free from weeds 
year after year—and if we dig this up in spring, rake the surface, and 
then leave it quite alone, what a number of plants soon make their 
appearance! They could be counted by hundreds, if not by 
thousands. And further, if we were to leave this patch of weeds 
alone, how many would come to maturity and perpetuate their own 
kind? Probably only a small percentage. A vast number are eaten 
over by slugs, and the innumerable insects and other little creatures 
which abound on all soil, others while still very young are choked out 
by their nearest neighbours, and of those which survive it is compa- 
ratively only a few which attain such robust growth as to ripen their 
seeds. And if we were to leave such a piece of ground untilled for a 
very few years, we should find some very remarkable changes in the 
species of plants growing on it. Probably at first ordinary weeds of 
cultivation, such as chickweed, mouse-ear chickweed, shepherd’s 
purse, white clover, sorrel, couch grass, sowthistle, &c., would 
spring up. But after three or four years it would be found that all 
these and the vast number of seeds which they spread on the ground 
would have largely given way to various species of grasses, the very 
existence of which would again depend on other conditions, such as— 
what animals were allowed to graze upon them,—whether the ground 
was trodden on,—its drainage, exposure, &c. Wallace has stated this 
law of population of species in his work on “ Natural Selection” as 
forcibly as Darwin has, and the following example is quoted from his 
words—“ Very few birds produce less than two young ones each 
year, while many have six, eight, or ten; four will certainly be below 
the average; and if we suppose that each pair produce young only 
four times in their life, that will also be below the average, supposing 
them not to die either by violence or want of food. Yet at this rate, 
how tremendous would be the increase in a few years from a single 
pair. A simple calculation will show that in fifteen years each pair 
of birds would have increased to nearly ten millions”—(in a note he 
says, “this is underestimated; the number would really amount to 
more than two thousand millions”)*—whereas we have no reason to 
believe that the number of birds in any country increases at all in 
fifteen or in one hundred and fifty years. With such powers of 
increase the population must have reached its limits, and have 
become stationary in a very few years after the origin of each species. 
It is evident, therefore, that each year an immense number of birds 
must perish—as many, in fact, as are born; and as, on the lowest | 
calculation, the progeny are cach year twice as numerous as their 
parents, it follows that, whatever be the average number of 
individuals existing in any given country, twice that numoder must 
perish annually,—a striking result, but one which seems, at least, 
highly probable.” If this statement is true of birds, and there is no 
reason to doubt its accuracy, how much greater must be the actual 
* Both numbers are incorrect; the exact figures are 26,028,192—or allowing 
for the deaths at the end of the fifteenth year 25,806,512, 
