,8,, EVOLUTION OF THE MAMMALIA. loi 



pendulum variations from the particular direction will be similarly 

 damped down. It will wobble a little, but its wobbling will be as 

 nothing compared with the swing that is fostered by selection. In 

 this case, then, selection will choose between the little more com- 

 plexity that is advantageous and the little less complexity that is 

 disadvantageous. The little less complexity will be eliminated, the 

 little more complexity will survive. The little less and the little 

 more, however, are in the same line of developmental swing. Hence, 

 the variations discoverable in fossil mammals in which tooth-develop- 

 ment along special lines is in progress, will, on the hypothesis of 

 selection, be plus and minus along a given line ; in other words, the 

 variations will be determinate, and in the direction of special 

 adaptive modifications. 



Of course, we may believe — as those who hold the view that 

 acquired characters are transmitted to offspring do believe — that indi- 

 vidual use gives the pendulum an added push. But if the considera- 

 tions above urged have any weight, we cannot prove this by a study 

 of palaeontological records. It must be proved, if proved at all, by 

 some crucial experiment still to be devised. 



Unquestionably the phenomena of parallelism on which Dr. 

 Scott, as we have seen, lays stress, forces on the believer in Natural 

 Selection as the sole known factor in evolution the assumption that 

 in different groups similar congenital variations occur. In the 

 language of our analogy, each of half a dozen different groups has 

 among its hundred pendulums one v/hich is swinging in such a way 

 that the plane is parallel to that of the other five. Or in the case 

 of prismatic and complex tooth-structure cited from AI. Kowalevsky, 

 horses, rhinoceroses, pigs, ruminants, elephants, and others are all 

 ready to develop a certain general type of molar, when the extension 

 of grassy plains renders such development of advantage. It is, 

 however, of the essence of the Darwinian faith to believe that the 

 whole organism is eminently plastic, and, according to Dr. 

 Weismann's view, the ever-varying combinations of germ-plasm 

 give unlimited possibilities of advance, retrogression, or divergence 

 in the congenital modification of structure. 



In conclusion, I may, perhaps, be allowed to say that in the 

 general tendency of our opinions Dr. Scott and I are at one. Where 

 we differ is in the estimate of the value of palaeontological evidence. 

 By itself, this does not seem to me to be sufficiently convincing or 

 conclusive to win over a single follower of Dr. Weismann. If the 

 transmission of acquired characters were demonstrated as an unques- 

 tionable fact, the Dynamic Theory would no doubt be generally 

 accepted as a vera causa in the evolution of the Tertiary Mammalia of 

 the North American Continent ; but to one who denies the possi- 

 bility of such transmission, all the facts brought forward by Dr. Scott 

 will still be held to be explicable on the hypothesis of Natural 

 Selection. C. Llovd Morgan. 



