It is a well-known observation that each great epoch of geological time has had its dominant forms of animal life, which, so far as the earth's history is known now, came into existence, lived their time, and were seen no more. In the same way the smaller groups of species and genera included in an ordinal group of animals or class have abounded, giving a tone to the life of each geological formation, until the vitality of the animal is exhausted, and the species becomes extinct or ceases to preponderate. This process is seen to be still modifying the life on the earth, when some kinds of animals and plants are introduced to new conditions. Plants appear to wage successful war more easily than animals. The introduction of the Cactus in some parts of Cape Colony has locally modified both the fauna and flora, just as the Anacharis introduced into England spread from Cambridge over the whole country, and became for many years the predominant form of plant life in the streams. The Rabbit in Australia is a historic pest. Something similar to this physical fertility and increase appears to take place under new circumstances in certain organs within the bodies of animals, by the development of structures previously unknown. A familiar example is seen in the internal anatomy of the Trout introduced into New Zealand, where the number of pyloric appendages about the stomach has become rapidly augmented, while the size and the form of the animal have changed. The rapidity with which some of these changes have been brought about would appear to show that Nature is capable of transforming animals more rapidly than might have been inferred from their uniform life under ordinary circumstances. Growth of the vital organs in this way may modify the distinctive form of any vital organ, brain or lungs, and thus as a consequence of modification of the internal structures due to changes of food and habit, bring a new group of animals into existence. And just as the group of animals ceases to predominate after a time, so there comes a limit to the continued internal development of vital structures as their energy fails, for each organ behaves to some extent like an independent organism.