EVIDENCES OF EVOLUTION 45 



were any mammals; similarly, the Eusta- 

 chian tube which runs past the ear to the 

 back of the mouth in amphibians, reptiles, 

 birds and mammals corresponds to the first 

 gill-cleft or spiracle of a shark. Begging 

 the question, we may state it as one of the 

 laws of evolution that markedly new struc- 

 tures have often arisen from the transfor- 

 mation of old structures of quite different 

 function. 



Classification. — Some reckon that there 

 are over a million different species of living 

 creatures, and, in any case, there are many 

 myriads. Now these species are, in many 

 cases, linked together by varieties which 

 make strict severance difficult. They are 

 like constellations, well-defined at first glance, 

 which on closer inspection are seen to be 

 connected by outlying members with ad- 

 jacent constellations. Moreover, they can 

 be rationally arranged in genera, orders, 

 families and classes; yet between these 

 there appear not a few remarkable connect- 

 ing links; there is structural progress from 

 the unicellular organisms upwards along 

 various lines of organization; and it is 

 possible to make a provisional genealogical 

 tree which is becoming less and less shadowy 

 every year, though the mutual relations of 

 the larger branches are still very obscure. 



