THE METHOD OF ORGANIC EVOLUTION. 421 



whole, may become quite discontinuous in relation to two species dif- 

 fering in very slight characters. Darwin dwelt much upon this phe- 

 nomenon of new species being- formed when any body of individuals 

 seized upon vacant places in the economy of nature, and by means of 

 comparatively slight variations became adapted to it. It is what we 

 see everywhere in the world around us. 



It thus api)ears that what is evidently supposed to be a very power- 

 ful argument, leading to the conclusion that discontinuous variations 

 as a class are those which are of vital, if not exclusive, importance in 

 the production of new species, entirely breaks down when confronted 

 with the facts of nature. It does not, however, follow that because an 

 unsound a priori argument has been used to call attention to these vari- 

 ations, and because they have been set before the world in a way to 

 suggest that their importance in relation to the origin of species is a 

 new discovery calculated to revolutionize the study of this branch of 

 biology, they are therefore of no value in this connection. We will 

 therefore now proceed to consider them on their own merits as possible 

 factors in the process of organic evolution. For this purpose we must 

 briefly indicate the nature of the variations so laboriously recorded in 

 this volume. 



Tiiese consist of what are termed meristic variations; that is, varia- 

 tions in the number or position of parts which occur in series, whether 

 linear, bilateral, or radial. Such are the variations in the number of 

 segments of annulosa and arthropoda, such as worms, leeches, centi- 

 pedes, etc.; in the antenna? and legs of insects; in the vertebne, ribs, 

 teeth, nipples, limbs, and toes of vertebrates; in the rays of starfish, 

 encrinites, and allied animals. The ocelli and other symmetrical mark- 

 ings on tlie wings of bntterflies are also recorded, as well as numerous 

 malformations when these affect serial or symmetrical organs. 



On carefully looking through the cases of variation in this volume, 

 we are struck with the largo })roportion of them which exhibit more or 

 less deformation or want of symmetry, culminating in the ^'arious kinds 

 of monstrosity. In Chai^ter III, on the variations of vertebra^ and ribs, 

 we find vertebra- imperfectly divided in snakes and frogs. Numerous 

 cases of abnormalities in human vertebra- are given, usually exhibiting 

 asymmetry or deformation, and similar variations are found in the 

 anthropoid ajies, but here there is apparently more of regularity and 

 symmetry. Tlie greatest amount of this kind of variation occurs in 

 the sloths, as might be expected when we consider that they are the 

 most abnormal of mammals as regards the cervical vertebra;. In Chap- 

 ter VIII immerous cases of superinimerary mamma^ are recorded, almost 

 all of which are unsynunetrical. The variations in the number or form 

 of the horns in sheep, goats, and deer recorded in Chapter XI show 

 them to be usually more or less irregular. 



Xearly a hundred pages are devoted to the digits (fingers and toes) 

 of mammals and birds, about one hundred and forty cases of variation 



