XXI.] EMBRYOLOGY. 263 



This law stands in the closest connexion with that 

 which we have seen to be a general law of comparative 

 morphology : I mean that those characters which clistin- characters 

 guish the widest groups are also the least liable to variation groupslre 

 as between orders, genera, species, or individuals within the least 

 group. The characters which appear first in the embryo ^^^^g^^'^^^ 

 are those of the widest group, and the characters of the of this 

 widest group are the least subject to variation. From ^^^'gy°^_ 

 these two laws it follows, by syllogistic inference, that the 

 characters which appear the earliest during development 

 are the least subject to variation. The reason of this is 

 tolerably evident. " If certain organs are formed early, Eeason. 

 those which come later must obviously accommodate them- 

 selves to their predecessors ; and any variations which 

 have taken place in the latter will perturb the normal 

 disposition of the former." ^ Thus, the first-formed parts 

 will vary only from such causes of variation as may arise 

 in themselves; but the later-formed ones will vary, not 

 ■ only from such causes as may arise within themselves, but 

 from any cause that may produce variation in their pre- 

 decessors : so that the later any part is formed, the more 

 chances it will have of varying. Whether this explanation 

 is satisfactory or not, a good instance of the fact is pre- 

 sented by the contrast between the development of the 

 MoUusca on the one hand, and that of the Articulata and 

 Vertebrata on the other. In the Articulata and Vertebrata, ^^^^J^Tkt'a, 

 the neural side of the body, or that containing the nervous and Ver- ' 

 centres, is developed before the haemal side, or that con- *^^^'^*^- 

 taining the circulatory centre ; and throughout those two 

 great groups the general form of the nervous system is 

 remarkably constant. In the MoUusca, on the contrary, 

 the heemal side is developed first, and the neural side after 

 it ; and among them the plan of the nervous system is 

 very much less constant.^ The MoUusca present another 

 remarkable instance of the same law. One of the most 

 conspicuous and best-known moUuscan characters is the 

 very iinsymmetrical form of their digestive organs, the 



1 Huxley on the Morphology of the Cephalous MoUusca, Philosophical 

 Transactions, 1853. ^ Ibid. 



