328 



SCIENCE 



[N. S. Vol. XXX. No. 767 



as we trace them from their origin to their 

 prime. It is well known that in certain of 

 the highest and latest types of bony fishes 

 the vertebrEe and fin-rays are reduced to a 

 fixed and practically invariable number 

 for each family or genus, whereas there is 

 no such fixity in the lower and earlier 

 groups. In the earliest known Pycnodont 

 fishes from the Lower Lias (Mesodon) the 

 grinding teeth form an irregular cluster, 

 while in most of the higher and later 

 genera they are arranged in definite regu- 

 lar rows in a symmetrical manner. Many 

 of the lower backboned animals have teeth 

 with several cusps, and in some genera the 

 number of teeth seems to be constant ; but 

 in the geological history of the successive 

 classes the tooth-cusps never became fixed 

 individual entities, readily traceable 

 throughout whole groups, until the highest 

 or mammalian grade had been attained. 

 Moreover, it is only in the same latest grade 

 or class that the teeth themselves can be 

 treated as definite units, always the same 

 in number (forty-four), except when modi- 

 fied by degeneration or special adaptation. 

 In the earlier and lower land animals the 

 number of vertebrae in the neck depends on 

 the extent of this part, whereas in the 

 mammal it is almost invariably seven, 

 whatever the total length may be. Curi- 

 ously constant, too, in the modern even- 

 toed hoofed mammals is the number of 

 nineteen vertebras between the neck and 

 the sacrum. 



I am therefore still inclined to believe 

 that the comparison of vital processes with 

 certain purely physical phenomena is not 

 altogether fanciful. Changes towards ad- 

 vancement and fiLsity which are so deter- 

 minate in direction, and changes towards 

 extinction which are so continually re- 

 peated, seem to denote some inherent prop- 

 erty in living things, which is as definite as 

 that of crystallization in inorganic sub- 

 stances. The regular course of these 



changes is merely hindered and modified 

 by a succession of checks from the environ- 

 ment and natural selection. Each separate 

 chain of life, indeed, bears a striking re- 

 semblance to a crystal of some inorganic 

 substance which has been disturbed by im- 

 purities during its growth, and has thus 

 been fashioned with unequal faces, or even 

 turned partly into a mere concretion. In 

 the case of a crystal the inherent forces act 

 solely on molecules of the crystalline sub- 

 stance itself, collecting them and striving, 

 even in a disturbing environment, to ar- 

 range them in a fixed geometrical shape. 

 In the case of a chain of life (or organic 

 phylum) we may regard each successive 

 animal as a temporary excrescence of col- 

 loid substance round the equally colloid 

 germ-plasm which persists continuously 

 from generation to generation. The inher- 

 ent forces of this germ-plasm, therefore, act 

 upon a consecutive series of excrescences 

 (or animal bodies), struggling not for geo- 

 metrically arranged boundaries, but 

 towards various other symmetries, and a 

 fixity in number of multiple parts. When 

 the extreme has been reached, activities 

 cease, and sooner or later the race is dead. 

 Such are some of the most important 

 general results to which the study of fos- 

 sils has led during recent years; and they 

 are conclusions which every new discovery 

 appears to make more certain. When we 

 turn to details, however, it must be ad- 

 mitted that modern systematic researches 

 are continually complicating rather than 

 simplifying the problems we have to solve. 

 Professor Charles Deperet has lately written 

 with scant respect of some of the pioneers 

 who were content with generalities, and 

 based their conclusions on the geological 

 succession of certain anatomical structures 

 rather than on a successive series of indi- 

 viduals and species obtained from the dif- 

 ferent layers of one geological section ; but 

 even now I do not think we can do much 



