660 LAND MAMMALS IN THE WESTERN HEMISPHERE 



has been remarkably direct and unswerving. Plasticity of 

 organization and capacity for differentiation of structure in 

 widely different directions would seem to be limited in the 

 mammals, especially among the more advanced groups. 



IV. A question that has been much debated and is still a 

 centre of controversy deals with continuity and discontinuity 

 in development. In other words, does evolution proceed by 

 the cumulative effects of minutely graded modifications, or 

 is it a succession of leaps and sudden changes? The differ- 

 ence is illustrated by many breeds and races of animals and 

 plants under domestication, the history of which is known. 

 Some have arisen from "sports," sudden and marked devia- 

 tions from the parent stock, which "breed true" from the 

 beginning. Of this character was the Ancon breed of sheep, 

 which was derived from a single short-legged ram that was 

 born of normal parents in 1791 and transmitted his peculiarities 

 to his offspring. Professor Castle's race of four-toed Guinea 

 Pig originated from one four-toed individual, which suddenly 

 appeared in a litter of normal ones. Other breeds have been 

 formed by the careful and long-continued selection of minute 

 individual variations. Which of these methods is the one that 

 has been followed under natural conditions? or has now one 

 method been used and now another, according to circumstances ? 

 The problem is one that has a profound and far-reaching im- 

 portance for the whole of evolutionary philosophy, which 

 largely hinges upon it. 



Unfortunately, palaeontology is not well fitted to give a 

 decisive answer to these questions, for, however complete the 

 record of any given series may be, we never can be sure that it 

 actually is so, and interruptions in the continuity of develop- 

 ment might be due either to progress by abrupt changes, or 

 to a failure to preserve all the gradations. For that reason 

 different observers have put divergent interpretations upon 

 the facts as we have them. The general impression that is 

 made by the study of a well-preserved mammalian phylum 



