^6 BIOLOGICAL LECTURES. 



and rendered more or less probable by this method, but one or 

 two principles which stand out with especial clearness may be 

 mentioned. 



(a) Evolution is ordinarily a continuous process of change 

 by means of small gradations. The continuous character of a 

 phylum is apt to be proportional to the relative abundance of 

 its representatives in the strata, which is equivalent to saying 

 that well-known series are continuous, while apparently dis- 

 continuous series are imperfectly known. This does not imply 

 that the rate of change was always uniform, it probably was 

 not, or that a sudden alteration of conditions may not bring 

 about discontinuity, or per saltum development. It means 

 that the usual and normal mode of advance is by continuity 

 of change. 



(b) Development is, in most instances, direct and unswerv- 

 ing. The rise of new forms, and the decadence and degen- 

 eration of old ones, are not ordinarily by zigzag and meandering 

 paths, but by relatively straight ones ; and though, of course, 

 a path once taken may be diverged from, yet in such a case it 

 is not regained. This applies particularly to the organism as 

 a whole ; in minor details more latitude is permissible. The 

 evidence is not yet sufficient to show just how widely applicable 

 this principle is. 



(c) Parallelism and convergence of development are much 

 more general and important modes of evolution than is com- 

 monly supposed. By parallelism is meant the independent 

 acquisition of similar structure in forms which are themselves 

 nearly related, and by convergence such acquisition in forms 

 which are not closely related, and thus in one or more respects 

 come to be more nearly alike than were their ancestors. 

 While some observers have tacitly or explicitly denied the 

 reality of these processes, most authorities have been com- 

 pelled to admit them. What palaeontology has done, and is 

 doing, is to show the universality of these modes of develop- 

 ment, and to point them out in directions where they had not 

 been suspected. To give a few examples. The crescentic, or 

 selenodont molar, has been separately acquired by no less than 

 three groups of artiodactyls, and probably others as well. The 



