284 STUDIES, SCIENTIFIC AND SOCIAL ch. xiii 



Let us now briefly summarise the general principles on 

 which the solution of problems in zoological distribution 

 depends. 



During the evolution of existing forms of animal life, we 

 may picture to ourselves the production of successive 

 types, each in turn increasing in variety of species and 

 genera, spreading over more or less extensive regions of the 

 earth's surface, and then after arriving at a maximum of 

 development, passing through various stages of decay, 

 dwindling to a single genus or a single species, and 

 finally becoming extinct. While the forms of life are 

 thus, each in turn, moving on from birth to maturity and 

 from maturity to decay and death, the earth's surface will 

 be undergoing important physical changes, which will 

 sometimes unite and sometimes separate contiguous 

 continents or islands leading now to the intermingling, noAV 

 to the isolation, of the progressing or diminishing groups 

 of animals. Again, we know that climates have often 

 changed over a considerable portion of the earth, so that 

 what was at one time an almost tropical region has become 

 at another time temperate, and then even arctic ; and 

 these changes have, it is believed, been several times 

 repeated, leading each time to important changes, 

 migrations, and extinctions of animal and vegetable life. 



It is by the combined effect of these three distinct sets 

 of causes, acting and reacting on each other in various 

 complex ways, that those curious examples of erratic 

 distribution of species and genera have been produced 

 w^hich have been so long a puzzle to the naturalist, but 

 which have now, it is believed, been shown to be the 

 natural and inevitable results of the process of animal 

 development, combined with constant changes in the 

 geography and in the climate of the earth. 



