THE PROBLEMS OF MORPHOLOGY. 9 



line forms. We have consequently to regard this proclivity 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 integrations 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 them- 

 selves. We may go through the several great groups of 

 organisms, with the view of reaching, by comparison of parts, 

 certain general truths respecting 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 investigations, we 

 may carry them on hand in hand — first establishing each 

 general truth empirically, and then proceeding to the ra- 

 tionale 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. 



[Note. — In preparation for treating of morphological de- 



