486 PHILOSOPHIC ZOOLOGY 



we may take the Field Vole {Arvicohx arva/is), which produces 

 several broods during the same summer, some of these propa- 

 o-atino' in their turn before winter. Crampe has calculated that, if 

 there were no checks to increase, a single pair of these animals, 

 supposing their first brood to be born on April 1 5 of a given 

 year, would be represented by the very respectable total of 198 

 on the following 8th of October. Continental agriculturists 

 sometimes have a very unpleasant object-lesson as to these 

 possibilities, for in certain "vole years" the ordinary checks to 

 increase are inefficient, the result being that enormous numbers 

 of field-voles make their appearance, and do an immense amount 

 of damage to crops. 



It not inirequently happens that when particular species of 

 animals are introduced into a new country, where the checks 

 that keep down their numbers in their native countries cease to 

 operate, the)- increase in a phenomenal way. The result of intro- 

 ducing rabbits into Australia affords one of the best examples 

 of this. 



Varl\tion and Hereditv. — It is a well-known fact that no 

 two individuals of the same species are precisely alike. There 

 is, in other words, a tendency to vary. The fact of Variation 

 enables us to understand why certain individuals, rather than 

 others, have a better chance of surviving in the struggle for 

 existence. For in any given environment variations in some 

 directions must more or less favour the animals which possess 

 them. They are, in fact, nsc/ii/ variations, tending to greater 

 fill/CSS as regards some particular set of surroundings. In many 

 herbivorous animals, for example, in regions where carni\'orous 

 enemies abound, it is clear that an indi\-idual \'ar)'ing in such a 

 way that its locomotor powers are somewhat better than those 

 of its fellows, will have a better chance of escaping from enemies, 

 and also of securing an abundant supply of food. Other things 

 being equal, it will also be more likely to perpetuate its species 

 than more slowly moving individuals of the same species. 



Next comes the question of Heredity. No one disputes the 

 possibility of certain characters being transmitted from one genera- 

 tion to another. The doctrine of Natural Selection invoh'es the 

 view that favourable variations are thus perpetuated, and as, in 

 each successive generation, individuals which continue to vary 

 in favourable directions will have the best chance of surviving, 



