26 WORLD-MAKING 



zoic until now, and modern species seem scarcely at all to 

 differ from specimens procured from rocks at least half-way 

 back to the beginning of our geological record. If we suppose 

 that the present sponges and foraniinifera are the descendants 

 of those of the Silurian period, we can affirm that in all that 

 vast lapse of time they have, on the whole, made little greater 

 change than that which may be observed in variable forms at 

 present. The same remark applies to other low animal forms. 

 In types somewhat higher and less variable, this is almost 

 equally noteworthy. The pattern of the venation of the wings 

 of cockroaches, and the structure and form of land snails, 

 gally-worms and decapod crustaceans were all settled in the 

 Carboniferous age, in a way that still remains. So were the 

 foliage and the fructification of club-mosses and ferns. If, at 

 any time, members of these groups branched off, so as to lay 

 the foundation of new species, this must have been a very rare 

 and exceptional occurrence, and one demanding even some 

 suspension of the ordinary laws of nature. 



We may perhaps be content on this question to say with 

 Gaudry, 1 that it is not yet possible to " pierce the mystery that 

 surrounds the development of the great classes of animals," or 

 with Prof. Williamson, 2 that in reference to fossil plants " the 

 time has not yet arrived for the appointment of a botanical 

 King-at-arms and Constructor of pedigrees." W r e shall, how- 

 ever, find that by abandoning mere hypothetical causes and 

 carefully noting the order of the development and the causes 

 in operation, so far as known, we may reach to ideas as to cause 

 and mode, and the laws of succession, even if unable to pene- 

 trate the mystery of origins. 



Another caution which a palaeontologist has occasion to give 

 with regard to theories of life, has reference to the tendency of 

 biologists to infer that animals and plants were introduced 



1 " Enchainements da Monde Animal/' Paris, 1883. 



2 Address before Royal Institution, Feb., 1883. 



