27 



At this point, I will ask your attention for some problems whose 

 pertinency to our discussion will be evident, anda n answer to which 

 will determine our action in questions yet to be considered. 



THE EFFECT OF TEMPERATURE ON DISTRIBUTION. 



Our experimental knowledge of the influence of temperature on 

 animal life in isolated cases prepares us to find it a largely control 

 ling factor in the geographical distribution of the various minor 

 types, and the thermo-physiology of many super-generic types is 

 in strict consonance with their morphology. Many of the forms 

 which have been noticed as characteristic of specific realms or com 

 mon to certain zones are limited in their range by thermometric 

 conditions. As a rule, temperature exerts a paramount influence 

 on the distribution of animal life. Nevertheless, the familiar adage 

 that there is no rule without an exception holds good for all the 

 laws of zoogeography. For instance, the tiger, which is as 

 sociated, in the minds of most persons, with the jungles of torrid 

 India, flourishes likewise in the frigid Amurland; the humming 

 birds, so characteristic of tropical America, are represented by 

 wanderers in the icy regions of both the north and south, and the 

 cyprinoid fishes live and multiply, with not greatly unequal facility, 

 in the waters of the torrid and frigid zones. Nevertheless, the rule 

 in general is that temperature is a most important factor in distri 

 bution, and probably to its influence, in connection with the former 

 distribution of animal types and the bounds of older continents, are 

 due some of the differences which are now so salient, as, for in 

 stance, between the North American and Eurasiatic realms. 



In former times, families now confined to America were also rep 

 resented in Europe ; such are the Snapping turtles or Chelydridae 

 and the Lepidosteidae. Perhaps their present absence from the Eu 

 ropean fauna is due to their former limitation southward, and the 

 cold of the Quaternary period, which extended over the whole of 

 their European domain and exterminated the species, while in 

 America, although the individuals over a large extent of territory 



