316 Dr. A. S. Woodsvaid— r//e Pidatlons of 



evi lution in a brief interval tlian has taken place during the 

 whole of the succeeding Tertiary time. In short, the funda- 

 mental advances in the grade of fish-life have always been 

 sudden and begun with excessive vigour at the end of a long 

 period of apparent stagnation, while each advance has been 

 marked by the fixed and definite acquisition of some new 

 character — an " expression point,^' as Cope termed it — which 

 seems to have rendered possible, or, at least, been an essential 

 accompaniment of, a fresh outburst of developmental energy. 

 As we have seen, the successive " expression points'^ among 

 fishes were the acquisition of (1) paddle-like paired fins, 



(2) shortened fin-bases but persistent heterocercal tail, 



(3) completed balancing fins and homocercal 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 lirdis would be much commoner in collections 

 than they actually are if the doctrine of gradual evolution 

 were correct. Paleeontology, indeed, is clearly in favour of 

 the theory of discontinuous mutation, or advance by sudden 

 changes, which has lately received so much support from the 

 botanical experiments 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 provided every kind of fish once more on a 

 higher plane or, so to speak, in a later fashion. The chang? 

 was inevitable and according to some fundamental law of 



