THE PROBLEMS OF MOUPHOLOGY. 9 



forms. We have consequently to regard this polarity of the 

 physiological units, as producing, during the development of 

 any organism, a combination of internal forces that expend 

 themselves in working out a structure in equilibrium with 

 the forces to which ancestral organisms were exposed ; but 

 not in equilibrium with the forces to which the existing organ- 

 ism is exposed, if the environment has been changed. Hence 

 the problem in all cases, is, to ascertain, the resultant of inter- 

 nal organizing forces, tending to reproduce the ancestral form, 

 and external modifying forces, tending to cause deviations from 

 that form. Moreover, we have to take into account, 



not only the characters of immediately-preceding ancestors, 

 but also those of their ancestors, and ancestors of all degrees of 

 remoteness. Setting out with rudimentary types, we have 

 to consider how, in each successive stage of evolution, the 

 structures acquired during previous stages, have been ob- 

 scured by further iategrations and further differentiations ; 

 or, conversely, how the lineaments of primitive organisms 

 have all along continued to manifest themselves under the 

 superposed modifications. 



§ 179. Two ways of carrying on the inquiry suggest 

 themselves. "We may go through the several great groups 

 of organisms, with the view of reaching, by comparison 

 of parts, certain general truths respectiug the homologies, 

 the forms, and the relations of their parts ; and then, having 

 dealt with the phenomena inductively, may retrace our steps 

 with the view of deductively interpreting the general truths 

 reached. Or, instead of thus separating the two inves- 

 tigations, we may carry them on hand in hand— first 

 establishing each general truth empirically, and then pro- 

 ceeding to the rationale of it. This last method will, I 

 think, conduce to both brevity and clearness. Let us now 

 thus deal with the first class of morphological problems. 



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