A4 



1 6 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 line (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 

 (^nt) and solid {U). The full theory 

 J^.— ^ of parallelism requires, not only that 

 "5 ^ 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 

 hnes : parallel development (the relation of m to b\ 

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

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

 when we recognize in places the absence of the facts we 

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

 — 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, 



