i6 



Psychophysical Evolution 



assumption of psychophysical research, may be repre- 

 sented by the accompanying diagram. The two vertical 

 lines (M, B) represent the two series in the evolution of the 

 race-forms of organisms — the dotted line (M, mind) being 

 the mental, and the solid Hne (B, body) the physical. 

 Across these at any point we may draw similar horizontal 

 lines {m, b) representing individual 

 development in any given genera- 

 tion ; these are also, of course, dotted 

 {m) and solid {b). The full theory 

 — _'ll.-_-^ of parallelism requires, not only that 

 I ^ we make the two horizontal lines 

 parallel, — the ordinary application 

 in ontogeny (O); but that having 

 gone so far, we must also draw the 

 two parallel vertical lines — the ap- 

 plication in phylogeny (P). At whatever point in the line 

 of descent we apply the principle to individual develop- 

 ment, we must perforce raise the corresponding genetic 

 questions about the evolution which has led up to the birth 

 of such individuals at that point. And the series of 

 * shorthand ' formulas, laws — in the prosaic equivalent 

 of everyday science, * results ' — at which we arrive, must 

 involve the three great problems represented by the four 

 lines : parallel development (the relation of m to b), 

 parallel evolution (the relation of M to B), and intcrgenetic 

 correlation (the relation of inb to MB). Furthermore, 

 when we recognize in places the absence of the facts we 

 should expect, — apparent breaks in either one of the lines, 

 — we may resort to the resource of using the correspond- 

 ing facts from the parallel line at the same level, and even 

 those from the analogous line of the other pair of parallels, 



