7^ SECTIONAL ADDB£SS£S. 



a saltation, and so giving a new centi-e to fluctuation and a fresh limit 

 to tlie upward transition. Those who maintain such an hypothesis 

 presumably regard ti'ansition as the response of the growing individual 

 body to gradual change of the physical environment (somatic modifica- 

 tion). But saltation they ascribe to a change in the composition of 

 the germ. That change may be forced on the germ by the condition 

 of the body, and may therefore be in harmony with the environment, 

 and may produce a new form along that line. The new form may be 

 obviously distinct from its predecessor, or the range of its fluctuation 

 may overlap that of its predecessor, in which case it will be impossible 

 to decide whether the change is one of transition or of saltation. This 

 succession of hypotheses involves a good many difficulties; among 

 others, the mechanism by which the germ is suddenly modified in 

 accordance with the transition of the body remains obscure. But the 

 facts before us seem to necessitate either perpetual transition or salta- 

 tion acting in this manner. Transition, we must admit, also involves 

 a change of the germ pari passu with the change of the body. Conse- 

 quently the difference between the two views seems to be narrowed 

 down to a point which, if not trivial, is at any rate minute. 



The particulai- saltation-hypothesis wliich I have sketched may 

 x'emind some hearers of the ' expression points ' of E. D. Cope. That 

 really was quite a different conception. Cope beheved that, in several 

 cases, generic characters, after persisting for a long time, changed 

 with relative rapidity. This took place when the modifications of adult 

 structure were pushed back so far prior to the period of reproduction 

 as to be transmitted to the offspring. The brief period of time during 

 which this rapid change occurred in any genus was an expression-point, 

 and was compared by Cope to the critical temperature at which a gas 

 changes into a liquid, or a liquid into a solid. Tlie analogy is not 

 much more helpful than Galton's comparison of a fluctuating form to 

 a rocking polyhedron, which one day rocks too much and topples over 

 on to another face. It is, however, useful to note Cope's opinion that 

 these points were ' attained without leaps, and abandoned without 

 abruptness. ' He did not believe that ' sports ' had ' any considerable 

 influence on the course of evolution ' (1887, ' Origin of Fittest,' pp. 39^ 

 79; 1896, ' Factors Org. Evolution,' pp. 24, 25). 



The Direction of Change: Seriation. 



The conception of connected change, whether by transition or hy 

 scarcely perceptible saltation, or by a combination of the two processes, 

 leads us to consider the Direction of the Change. 



Those who attempt to classify species now living frequently find 

 that they may be arranged in a continuous series, in which each species 

 differs from its neighbours by a little less or a little more; they find 

 that the series corresponds with the geographical distribution of the 

 species; and they find sometimes that the change affects particular 

 genera or families or orders, and not similar assemblages subjected, 

 apparently, to the same conditions. They infer from this that the 

 series represents a genetic relation, that each successive species is the 

 descendant of its preceding neighbour; and in some caser; thi? inference 



