284 Proceedings of the Royal Physical Society. 



shorter period. As a direct bearing on some of these points, 

 the following remarks by Professor Huxley are of much 

 interest : — 



" If you divide the animal kingdom into orders, you will 

 find that there are above one hundred and twenty. The 

 number may vary on one side or the other, but this is a fair 

 estimate. . . . Among the Mammalia and birds there are 

 none extinct ; but when we come to the reptiles, there is a 

 most wonderful thing: out of the eight orders, or there- 

 abouts, which you can make among reptiles, one-half are 

 extinct. ... If we turn to the Amphibia, there was one 

 extinct order, the Labyrinthodonts. 



" No order of fishes is known to be extinct.^ Every fish 

 that we find in the strata — to which I have been referring — 

 can be identified and placed in one of the orders which exist 

 at the present day. There is not known to be a single 

 ordinal form of insect extinct. There are only two orders 

 extinct among the Crustacea. There is not known to be an 

 extinct order of these creatures, the parasitic and other 

 worms ; but there are two, not to say three, absolutely 

 extinct orders of this class, Echinodermata ; out of all the 

 orders Coelenterata and Protozoa only one, the Eugose Corals. 



" So that, you see, out of somewhere about one hundred and 

 twenty orders of animals, taking them altogether, you will not, 

 at the outside estimate, find above ten or a dozen extinct. 

 Summing up all the order of animals which have left remains 

 behind them, you will not find above ten or a dozen which 

 cannot be arranged with those of the present day ; that is to 

 say, that the difference does not amount to much more than 

 10 per cent. ; and the proportion of extinct orders of plants 

 is still smaller " (" Collected Essays," vol. ii. pp. 353-55). 



There is another principle bearing on the variation of 

 both plants and animals, which I cannot do better than 

 introduce to you in the words of Professor Eay Lankester : — 

 " Any new set of conditions occurring to an animal which 



^ As my friend Dr Traquair tells me, the classification of fishes has altered 

 since Huxley wrote these words. The " Ganoidei " are no longer esteemed 

 an order, and the Ostracodermi {Cejjhalaspis, Pterasjns, etc.), which used to 

 be classed as Ganoids, form an absolutely extinct order, or rather sub-class. 



