384 SCOTT. [Vol. V. 



comes out most clearly in the series of fossil mammals. Granting 

 that unlimited variation is no necessary part of the selection theory, 

 it seems strange that new facets on the bones and new cusps 

 on the teeth should appear only in such definite ways, and that 

 there should not be many tentative attempts and false starts 

 before the proper development is hit upon. In the structure 

 of the carpus and tarsus we find that in any given phylum 

 very definite lines of evolution are early established and closely 

 adhered to, and the changes are just those called for by the 

 operation of dynamical influences. If Weismann's position were 

 correct, we ought to find much greater latitude of variation. 



It is thus difficult to understand how even a single variation 

 can be established by the unaided operation of sexual reproduc- 

 tion and natural selection, but the difficulty is greatly increased 

 when we take into account the phenomena of parallelism and 

 convergence. We have seen that the selenodont type of molar 

 tooth has been independently acquired in at least three lines, 

 and probably in many more. The molariform premolars which 

 occur in so many perissodactyls, in some artiodactyls, and in 

 other groups, though copying more or less exactly the pattern 

 of the molars, are composed of parts which are not homologous 

 with those occupying similar positions in the molar crowns. 

 These changes can be followed step by step and accounted for, 

 if we admit the action of mechanical influences, but are very 

 puzzling on the hypothesis of all round variation. The spout- 

 shaped odontoid process of the axis occurs in the camels, the 

 true ruminants, the horses, and other groups, and it has been 

 shown that this structure is not due to inheritance from a com- 

 mon ancestor, but has been separately acquired. Still more 

 striking is the case cited by Kowalevsky (No. 26), of the sudden 

 and simultaneous appearance (using these terms in their geolog- 

 ical sense) in so many different groups of the prismatic or root- 

 less molar, with very complex enamel foldings, and the valleys 

 filled with cement. Kowalevsky believes that the occasion of 

 this remarkable revolution, as it may be fairly called, was the 

 great extension of the grassy plains, and the consequent change 

 on the part of most herbivorous animals from a diet of soft 

 plants and leaves, to the siliceous grasses. Now what is the 

 probability that such a series of changes in horses, rhinoc- 

 eroses, pigs, ruminants, elephants, and other families, should be 



