854 PALEONTOLOGY AND EVOLUTION xi 



lizard to the bird was effected. These, very prob- 

 ably, are still hidden from us in the older for- 

 mations. 



Let us now endeavour to find some cases of 

 true linear types, or forms which are intermediate 

 between others because they stand in a direct 

 genetic relation to them. It is no easy matter to 

 find clear and unmistakable evidence of filiation 

 among fossil animals; for, in order that such 

 evidence should be quite satisfactory, it is necessary 

 that we should be acquainted with all the most 

 important features of the organisation of the 

 animals which are supposed to be thus related, and 

 not merely with the fragments upon which the 

 genera and species of the palaeontologist are so 

 often based. M. Gaudry has arranged the species 

 of HycenidcB, Prohoscidea, Bhinccerotidce, and Equidce 

 in their order of filiation from their earliest appear- 

 ance in the Miocene epoch to the present time, and 

 Professor Riitimeyer has drawn up similar schemes 

 for the Oxen and other Ungulata — with what, I 

 am disposed to think, is a fair and probable approxi- 

 mation to the order of nature. But, as no one is 

 better aware than these two learned, acute, and 

 philosophical biologists, all such arrangements 

 must be regarded as provisional, except in those 

 cases in which, by a fortunate accident, large 

 series of remains are obtainable from a thick and 

 widespread series of deposits. It is easy to 

 accumulate probabilities — hard to make out somB 



