18 



EVOLUTION AND ANIMAL LIFE 



mass of shells, there a piece of petrified wood, an insect in the 

 marl bed or a leaf preserved flat in the shale. Each of these 

 fossils is a record of past life, true beyond impeachment, but 

 the fragments are so few, so scattered, so broken, as to give 

 only hints of the history they represent. 



Moreover, as we extend our studies of species we find that 

 they change with space as well as with time. These changes 



Fig. 7. — Common lizard or swift, Sceloporus undulatus. (Photograph by W. H. Fisher.) 



are in large degree a response to external conditions. As 

 conditions change, so do forms change to fit their surroundings. 

 A movement over the surface of the earth, any movement in 

 space, brings organisms in contact with barriers. A barrier 

 means a change in conditions of life. As distance in space 

 brings barriers, so does the passage of time bring events which 

 are barriers also, Time brings. new events; events mean 

 changes in conditions, and change brings about divergence. 

 Neither time nor space flows evenly. 



Variations in turn ^become greater with lapse- of time and 



