XVII] THE COMPARISON OF RELATED FORMS 1037 



wEich we cannot find, when we come to look for it, any strict 

 dividing line even between the head and the body, the muscle and 

 the tendon, the sinew and the bone. Characters which we have 

 differentiated insist on integrating themselves again; and aspects 

 of the organism are seen to be conjoined which only our mental 

 analysis had put asunder. The coordinate diagram throws into 

 rehef the integral solidarity of the organism, and enables us to see 

 how simple a certain kind of correlation is which had been apt to 

 seem a subtle and a complex thing. 



But if, on the other hand, diverse and dissimilar fishes can be 

 referred as a whole to identical functions of very different coordinate 

 systems, this fact will of itself constitute a proof that variation has 

 proceeded on definite and orderly line?, that a comprehensive "law 

 of growth" has pervaded the whole structure in its integrity, and 

 that some more or less simple and recognisable system of forces 

 has been in control. It will not only shew how real and deep-seated 

 is the phenomenon of "correlation," in regard to form, but it will 

 also demonstrate the fact that a correlation which had seemed too 

 complex for analysis or comprehension is, in many cases, capable of 

 very simple graphic expression. This, after many trials, I believe to 

 be in general the case, bearing always in mind that the occurrence of 

 independent or localised variations must sometimes be considered. 



We are deahng in this chapter with the forms of related organisms, in order 

 to shew that the differences between them are as a general rule simple and 

 symmetrical, and just such as might have been brought about by a slight and 

 simple change in the system of forces to which the living and growing organism 

 was exposed. Mathematically speaking, the phenomenon is identical with one 

 met with by the geologist, when he finds a bed of fossils squeezed flat or other- 

 wise symmetrically deformed by the pressures to which they, and the strata 

 which contain them, have been subjected. In the first step towards fossilisation, 

 when the body of a fish or shellfish is silted over and buried, we may take it 

 that the wet sand or mud exercises, approximately, a hydrostatic pressure — 

 that is to say a pressure which is uniform in all directions, and by which the 

 form of the buried object will not be appreciably changed. As the strata 

 consolidate and accumulate, the fosail Organisms which they contain will 

 tend to be flattened by the vast superincumbent load, just as the stratum 

 which contains them will also be compressed and will have its molecular 

 arrangement more or less modified*. But the deformation due to direct 

 vertical pressure in a horizontal stratum is not nearly so striking as are the 

 deformations produced by the oblique or shearing stresses to which inclined 



* Cf. Sorby, Quart. Journ. Geol. Soc. (Proc), 1879, p. 88. 



