562 PALEONTOLOGY 



persistent heterocercal tail, (3) completed balancing-fins and homo- 

 cereal tail, and (4) completed internal skeleton. 



When fossils are examined more closely, it is interesting to observe 

 that the geological record is most incomplete exactly at these critical 

 points in the history of each race. There are abundant remains of the 

 families and genera which are definitely referable to one or other 

 order or suborder; but with them there are scarcely any of the links 

 between these major divisions which might have been expected to 

 occur. It must also be confessed that repeated discoveries have now 

 left faint hope that exact and gradual links will ever be forthcoming 

 between most of the families and genera. The "imperfection of the 

 record," of course, may still render some of the negative evidence 

 untrustworthy; but even approximate links would be much com- 

 moner in collections than they actually are, if the doctrine of gradual 

 evolution were correct. Paleontology, indeed, is clearly in favor of 

 the theory of discontinuous mutation, or advance by sudden changes, 

 which has lately received so much support from the botanical experi- 

 ments of H. de Vries. 



Further results obtained from the study of fossils have a bearing 

 even on the deepest problems of biology, namely, those connected with 

 the nature of life itself. For instance, it is allowable to infer, from 

 the statements already made, that the main factor in the evolution of 

 organisms is some inherent impulse the " bathmic force" of Cope 

 which acts with unerring certainty, whatever be the conditions of the 

 moment. So far as human judgment can decide, the varied assemblage 

 of fishes at each stage of the earth's history was always in perfect 

 accord with its environment, and displayed very few signs of waning, 

 even at the time when a new race suddenly took its place and pro- 

 vided every kind of fish once more on a higher plane or, so to speak, in 

 a later fashion. The change was inevitable and according to some 

 fundamental law of life whose influence is independent of temporary 

 equilibrium. Equally inevitable and irreversible are the essential 

 changes which may be observed during the evolution of each family 

 of organisms. As the late Professor Beecher pointed out, 1 all animals 

 with skeletons tend to produce a superfluity of dead matter which 

 accumulates in the form of spines as soon as the race to which they 

 belong has passed its prime and begins to be on the down grade; all 

 vertebrates tend to lose their teeth when they reach the culmination 

 of their life-history ; nearly all groups of fishes end their career with 

 eel-shaped representatives; and when a structural character has been 

 definitely lost in the course of evolution it never reappears, but, if 

 actually wanted again, is reproduced in a secondary makeshift. 

 Finally, and perhaps most important of all, there is in the course of 



1 C. E. Beecher, The Origin and Significance of Spines, Amer. Journ. Sci. [4] 

 vol. vi (1898), July to October. 



