FACTORS IN ANIMAL LIFE 
Romanes found that rats would get certain semi- 
liquid foods out of a bottle with their tails, as a cat 
will get milk out of a jar with her paw, but neither 
ever progresses so far as to use any sort of tool 
for the purpose, or to tip the vessel over. Animals 
practice concealment to secure their prey, but not 
deception, as man does. They do not use lures or 
disguises, or traps or poison. 
There is, of course, no limit to the variety and 
adaptiveness of nature taken as a whole, but each 
species is hedged about by impassable limitations. 
The ouzel is akin to the thrushes, and yet it lives 
along and in the water. Does it ever take to the 
fields and woods, and live on fruit and land-insects, 
and nest in trees like other thrushes? So with all 
birds and beasts. They vary constantly, but not in 
one lifetime, and the sum of these variations, accu- 
mulated through natural selection, as Darwin has 
shown, gives rise, in the course of long periods of 
time, to new species. 
As I have already said, domestic animals vary 
more than wild ones. Every farmer and poultry- 
grower knows that some hens are better with chick- 
ens than others — more motherly, more careful — 
and rear a greater number of their brood. The same 
is true of sows with pigs. Some sows will eat their 
pigs, and wild animals in cages often destroy their 
young. Some ewes will not own their lambs, and 
occasionally a cow will not own her calf. (Such cases 
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