368 JOURNAL OF SCIENCE. 
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What constitutes a favourable and what an unfavourable 
variation is a point—as I have already said—that we are not always 
able to decide, nor can we tell beforehand how any plant or animal 
will be affected by a change in its environment. Suppose a number 
of cattle were placed on a small well-grassed island, utterly destitute 
of other grazing animals, their numbers in a few years would 
probably become stationary, but it would be difficult to say before- 
hand what characteristics would distinguish the survivors. Most 
likely those which grazed nearest to the ground would be enabled to 
live, while others died of want. But if again, sheep were to be intro- 
duced into the island, it would only be a question of a few years when 
there would be no cattle left. But if now, rabbits were introduced, 
the sheep in their turn would be exterminated. But suppose, to 
carry the idea further, that while some cattle were still left, and the 
sheep were increasing, rabbits and large hawks were simultaneously 
introduced—it is most likely that cattle, sheep, and rabbits would all 
continue to exist, but none of them able to exterminate the others, 
for while the sheep and rabbits would keep down the general supply 
of food for the cattle, they would themselves be kept in check by the 
hawks. Similar suppositious cases might be indefinitely extended, 
but it will be enough to show that the principle is equally potent in 
the case of plants. This is partly shown by the spread of naturalized 
- plants in this country. Our cultivated plants such as wheat, barley, 
oats, potatoes, &c., thrive and grow strongly under the artificial 
conditions of the farm, but as soon as the cultivator’s care is with- 
drawn, they succumb to plants of naturally far hardier constitution. 
They never become naturalized in the proper sense of the word. 
The very weeds of cultivation such as chickweed, spurrey, corncockle, 
sorrel, &c., which can fight the farmer and his crops so well, have no. 
chance if left to themselves. They only thrive under the tillage which 
results from breaking up the soil, and many of them being annuals, 
can spring up, scatter their seeds, and thus fulfil their cycle of 
existence before the corn has shot into ear. It is possibly the case 
that these annual weeds are themselves a class of plants whose special 
characteristics have been evolved since the time that men took to 
cultivating the ground. And if ground covered by them be left open 
to grazing animals, they will all disappear and their place be taken 
by other plants more suited to the altered conditions. All these and 
similar facts point irresistably to the conclusion that all plants and 
animals are engaged in a continual struggle for existence. In this 
struggle all those which survive to maturity do so by virtue of their 
adaptability to their surrounding circumstances, but when from any 
cause a variation arises in an organism which gives it an advantage 
over its fellows of the same species, that variation will tend to be 
reproduced and intensified by the action of a natural selection. 
4. I now come to another class of evidence in favour of the 
evolutionist doctrine of descent, namely, to the consideration of 
rudimentary organs. These were a great difficulty to the early 
naturalists, for they could make nothing out of their occurrence. 
They are very familiar to all, so I need only illustrate them generally 
by a few examples, referring more fully to a few particular cases 
among plants. There are numerous cases of ruminating animals, ¢.g., 
our common cattle, which have teeth in the jaws of the yet unborn 
calf, which never develope, and are, therefore, purposeless. Similarly, 
