Continuity of Land and Air 257 
varieties of animals that had been developed in them would 
not, and their places would be taken by aquatic forms from 
other areas, so that the number of species would be thereby 
greatly reduced, and wide-spreading forms would be freed 
from the competition of many improved varieties. 
Viewed in this light, the similarity of fresh-water produc- 
tions all over the world, instead of being a difficulty in the 
way of the acceptance of the theory of natural selection, 
becomes a strong argument in favour of its truth; for we 
perceive that the number of marine, terrestrial, and fresh- 
water animals is in proportion to the more or less continuous 
development that was possible under the different conditions 
tinder which they lived. 
The same line of argument might be used to explain the 
much greater variety in some classes of terrestrial animals 
than in others. The land has often been submerged in 
geological history, and the classes that were best fitted to 
escape the impending catastrophes would be most likely to 
preserve the varieties that had been developed. The atmo- 
sphere has always been continuous, and the animals that 
could use it as a highway had great advantages over those 
that could not, and so we find the slow-moving terrestrial 
mollusks few in number compared with the multitudinous 
hosts of strong-flying insects; similarly, the mammals are 
far outnumbered by the birds of the air, that can pass from 
island to island, and from country to country, unstopped by 
mighty rivers or wide arms of the sea. 
