190 MORPHOLOGICAL DEVELOPMENT. 



and various other parts, corresponding with its two-sidec 

 attitude in moving through the water. And in other genen 

 of this group, as in Cestum, Eurhamphcea, and Callianira, 

 that almost equal distribution of parts which characterize 

 the Beroe is quite lost. 



Here seems a fit place to meet the objection which some 

 may feel to this and other such illustrations, that they amount 

 very much to physical truisms. If the parts of a Medusa are 

 disposed in radial symmetry round the axis of motion through 

 the water, there will of course be no means of maintaining 

 one part of its edge uppermost more than another; and the 

 equality of conditions may be ascribed to the radiateness, as 

 much as the radiateness to the equality of conditions. Con- 

 versely, when the parts are not radially arranged around the 

 axis of motion, they must gravitate towards some one atti- 

 tude, implying a balance on the two sides of a vertical plane 

 — a bilateralness ; and the two-sided conditions so necessi- 

 tated, may be as much ascribed to the bilateralness as the 

 bilateralness to the two-sided conditions. Doubt- 



less the form and the conditions are, in the way alleged, 

 necessary correlates; and in so far as it asserts this, the ob- 

 jection harmonizes with the argument. To the difficulty 

 which it at the same time raises by the implied question — 

 Why make the form the result of the conditions, rather than 

 the conditions the result of the form? the reply is this: — 

 The radial type, both as being the least differentiated type 

 and as being the most obviously related to lower types, must 

 be taken as antecedent to the bilateral type. The indi- 

 vidual variations which incidental circumstances produce in 

 the radial type, will not cause divergence of a species from 

 the radial type, unless such variations give advantages to the 

 individuals displaying them ; which there is no reason to sup- 

 pose they will always do. Those occasional deviations from 

 the radial type, which the law of the instability of the homo- 

 geneous warrants us in expecting to take place, will, however, 

 in some cases be beneficial; and will then be likely to estab- 



