14 THOMAS D WIGHT ON THE 



not as o-ood as its nature admits. It ma}- be urg'ed that an essential element of per- 

 fection is the due proportion of parts and faeukies, and that it is impossible for ns to 

 say that greater development in some particular direction would be for the welfare of 

 the individual. I have no desire to dispute the great truth underlying these propositions, 

 but in view of rudimentary organs alone, it would ap])ear that at least in some subordi- 

 nate details useless struetm-es occur, and we have no means of deciding what limits to 

 assign to the action of the causes pi-oducing them. It is certain that these appearances 

 are not due to chance ; there must be some determining cause modifying the structure 

 in this direction. It is customary now to quote rudimentary organs and anatomical 

 anomalies as evidences of descent, but it seems t<j me very impi'0])erly, occurring as 

 many of them do quite out of the line of inhei'itance. This criticism applies with great- 

 est force to anomalies, but rudimentary organs seem phenomena of the same general 

 class. What principle then shall account for them? We must turn to homologies. 

 We see throughout vertebrates a general plan; and though great modifications occur, 

 the plan persists. No liberties, so to speak, are taken with it beyond a certain point. 

 There are, never, for instance, more than two eyes, or one mouth or four pairs of limbs. 

 Is there reason to be sure that none of these, or analogous modifications, might not be 

 for the benefit of the individual? Yet, I tliink every student of natural history would 

 look on the suggestion that they will ever occur as chimerical. 



The principle of homology has a restraining influence on variations of structure, both 

 in quasi-accidental instances, as in anomalies, and in transformation, of species (if that 

 occurs) by restricting changes within the limits of the general plan. 



To condense further these deductions, it appears that the internal structure of any 

 particular bone may show evidence of three factors: first, that of teleology; second, that 

 of homology ; third, that of correlation to the structure of other bones of the same ani- 

 mal. The relative prominence of these factors varies greatly. For example, in the hu- 

 merus of the whale the first is of little moment and so is the third in the heel bone of 

 the seal. Thus, while we find provision for the fitness of the part, we find also some- 

 times appai-ently useless structures, sometimes apparently evidences of degeneration, 

 but throughout ai'e more or less distinct marks of hai-mony with other parts, and of 

 homology with other forms. How has this been accomplished? Cleai'ly the crude 

 notion that accidental, purposeless, external forces should be sufficient to change by slow 

 degrees one such organism into another of a different species is untenable. The doc- 

 trine of chances alone shows it to be impossilile. There is, moreover, the unanswerable 

 argument of the inevitable uselessness of incipient structures. Where we see the need, 

 we see the structure to meet it already perfect. We see also the combination of ho- 

 mology with teleology. Whatever, therefore, the share of evolution maybe in the pro- 

 duction of species, it is not one of chance. The changes must be for the most part 

 comparatively sudden, and, therefore, due to an implanted, internal force acting in pre- 

 determined directions. On the theory of external accidental forces, the preservation of 

 homology is incomprehensible. The action, however, of this internal force, is, no doubt, 

 modified by accidental secondary causes, which may produce degenerative as well as 

 progressive changes. 



