ON THE HYPOTHESIS OF EVOLUTION. 137 



near approach to birds made by the reptiles Lselaps and Megadac- 

 tylus, and the combination of characters of the old genera Ich- 

 thyosaurus and Plesiosaurus in the Polycotylus of Kansas. 



We had no more reason to look for intermediate or connectins: 

 forms between such types as these than between any others of 

 similar degree of remove from each other with which we are ac- 



period represented by the fossiliferous rocks ; that it shows no evidence of such 

 modification ; and, as to the nature of that modification, it yields no evidence what- 

 soever tliat tlie earlier members of any long-continued group were more generalized 

 in structure than the later ones." 



Respecting this position, he says : " Thus far I have endeavored to expand and 

 enforce by fresh arguments, but not to modify in any important respect, the ideas 

 submitted to you on a former occasion. But when I come to the propositions re- 

 specting progressive modification, it appears to me, with the help of the new light 

 which has broken from various quarters, that there is much ground for softening 

 the somewhat Brutus-like severity with which I have dealt with a doctrine for the 

 truth of which I should have been glad enough to be able to find a good foundation 

 in 1862. So far indeed as the Invertebrata and the lower Vertebrata are concerned, 

 the facts and the conclusions which are to be drawn from them appear to me to re- 

 main what they were. For anything that as yet appears to the contrary, the earliest 

 known Marsupials may have been as highly organized as their living congeners ; the 

 Permian lizards show no signs of inferiority to those of the present day ; the laby- 

 rinthodonts can not be placed below the living salamander and triton ; the Devo- 

 nian ganoids are closely related to polypterus and lepidosiren." 



To this it may be replied: 1. The scale of progression of the Vertebrata is 

 measured by the condition of the circulatory system, and in some measure by the 

 nervous, and not by the osseous : tested by this scale, there has been successional 

 complication of structure among Vertebrata in time. 2. The question with the 

 evolutionist is, not what types have persisted to the present day, but the order in 

 which types appeared in time. 3. The Marsupials, Permian saurians, labyrintho- 

 donts, and Devonian ganoids are remarkably generalized groups, and predecessors of 

 types widely separated in the present period. 4. Prof. Huxley adduces many such 

 examples among the mammalian subdivisions in the remaining portion of his lect- 

 ure. 5. Two alternatives are yet open in the explanation of the process of evolu- 

 tion : since generalized types, which combine the characters of higher and lower 

 groups of later periods, must thus be superior to the lower, the lower must (first) be 

 descended from such a generalized form by degradation ; or (second) not descended 

 from it at all, but from some lower contemporaneous type by advance ; the higher 

 only of the two being derived from the first-mentioned. The last I suspect to be 

 a true explanation, as it is in accordance with the homologous groups. This law 

 will shorten the demands of paleontologists for time, since, instead of deriving all 

 Reptiiia, Batrachia, etc., from common origins, it points to the derivation of higher 

 Reptilia of a higher order from higher Reptiiia of a lower order, lower Reptiiia of 

 the first from lower Reptiiia of the second ; finally, the several groups of the lowest 

 or most generalized order of Reptiiia from a parallel scries of the class below, or 

 Batrachia. 



