46 EVOLUTION 



and yet its links were forged long before there 

 were any mammals ; similarly, the Eustachian 

 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 structures have often 

 arisen from the transformation of old struc- 

 tures 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 con- 

 stellations, well-defined at first glance, which 

 on closer inspection are seen to be connected 

 by outlying members with adjacent constella- 

 tions. Moreover, they can be rationally 

 arranged in genera, orders, families and 

 classes; yet between these there appear not a 

 few remarkable connecting links ; there is 

 structural progress from the unicellular organ- 

 isms upwards along various lines of organiza- 

 tion; 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 



