68 General Part. 



taken to a small island near California. They multiplied very rapidly, 

 and ten years later numerous descendants inhabited the island, but 

 the weight of the specimens had fallen to one-third of that of 

 the birds introduced ; in the course of a few generations they had 

 assumed a pigmy form. The effect of island life generally appears to 

 result in the formation of dwarfs. Changes in other respects have 

 resulted from the removal of animals into new surroundings. A 

 peculiar form of Wild Rabbit has been produced in course of time 

 in the island of Porto Santo ; it seems to be a distinct species, possess- 

 ing special colour-markings, etc. This has sprung from Spanish Rabbits, 

 which were imported 400 or 500 years ago. Trout were introduced 

 into New Zealand about twenty years ago ; their progeny varies now 

 in certain points in the operculum, from the European species. 



The influence of different external conditions upon the organism 

 is very clearly exhibited in domestic animals. Many of the 

 peculiarities exhibited by domestic races are simply the product of 

 local conditions, special food, etc. 



It should be pointed out that the nature of the above explanations 

 is unintelligible. Why Artemia should undergo modifications, as a 

 result of migrating into fresh water : why pigment should be deve- 

 loped in Proteus when it is exposed to the light : is not as yet exactly 

 understood. These facts are, however, of great importance, since they 

 demonstrate that the peculiarities of different faunas, the adaptation to 

 different kinds of food, the modification and degeneration of parasites, 

 mentioned in the preceding paragraphs, are in all probability, at 

 least to a great extent, the effects of external causes. 



5. The Stages of Life— Duration of Life. 



A number of stages may easily be distinguished in the life of 

 most animals. The first stage is the embryonic period; a second, the 

 ensuing time of youth ; a third, the period of complete development ; 

 and this, finally, may be followed by a state of retrogression. 



The embryonic period has already (p. 50) been sufficiently de- 

 scribed. The time of youth extends from birth until the animal 

 becomes sexually mature {i.e., produces ripe ova or spermato- 

 zoa), and therewith has almost attained its ultimate size and form. 

 During the adult period development is almost stationary, and the 

 organism passes quite gradually into the senile period, when the 

 organs suffer partial degeneration, and are functionally less active. 

 The whole strength of the organism, moreover, fails, so that it readily 

 falls a victim to adverse influences. This state is, however, only 

 clearly seen amongst the higher Vertebrata (Mammalia and Aves) . It 

 need scarcely be said that the different phases are by no means 

 sharply demarcated. 



