XVII] THE COMPARISON OF RELATED FORMS 743 



and have inscribed it in a rectangular net, with, abscissae three- 

 fifths the length of the ordinates. Side by side (Fig. 368) is drawn 

 a very different Copepod, of the genus Sapphirina; and about 

 it is drawn a network such that each co-ordinate passes (as nearly 

 as possible) through points corresponding to those of the former 

 figure. It will be seen that two differences are apparent. (1) The 

 values of y in Fig. 368 are large in the upper part of the figure, and 

 diminish rapidly towards its base. (2) The values of x are very 

 large in the neighbourhood of the origin, but diminish rapidly as 

 we pass towards either side, away from the median vertical axis; 

 and it is probable that they do so according to a definite, but 

 somewhat complicated, ratio. If, instead of seeking for an 

 actual equation, we simply tabulate our values of x and y in the 

 second figure as compared with the first (just as we did in com- 

 paring the feet of the Ungulates), we get the dimensions of a net 

 in which, by simply projecting the figure of Oithona, we obtain 

 that of Sapphirina without further trouble, e.g. : 



X {Oithona) 



x' {Sappliirina) 



y {Oithona) 



y' {Sapphirina) 



In this manner, with a single model or type to copy from, we 

 may record in very brief space the data requisite for the production 

 of approximate outlines of a great number of forms. For instance 

 the difference, at first sight immense, between the attenuated 

 body of a Caprella and the thick-set body of a Cyamus is obviously 

 little, and is probably nothing, more than a difference of relative 

 magnitudes, capable of tabulation by numbers and of complete 

 expression by means of rectilinear co-ordinates. 



The Crustacea afford innumerable instances of more complex 

 deformations. Thus we may compare various higher Crustacea 

 with one another, even in the case of such dissimilar forms as a 

 lobster and a crab. It is obvious that the whole body of the 

 former is elongated as compared with the latter, and that the 

 crab is relatively broad in the region of the carapace, while it 

 tapers off rapidly towards its attenuated and abbreviated tail. 

 In a general way, the elongated rectangular system of co-ordinates 



