8i6 THE POPULAR SCIENCE MONTHLY. 



theory I propose gives renewed importance to the Lamarckian 

 factors by showing one manner of their action not previously 

 urged, and it also helps us to a tangible and scientific conception 

 of design. 



Acceleration and Retardation. — In this rapid glance at the im- 

 mediate causes of variation we have discussed some factors which 

 in some degree represent laws rather than inducing causes of 

 variation. This difficulty appertains to all attempts at formula- 

 tion of the causes of variation, and only as our actual knowledge 

 increases shall we be able succinctly and definitely to classify the 

 factors. There are, however, certain important laws which have in- 

 fluenced modification but in no sense can be looked upon as causes 

 of variation. They are laws or principles of evolution, by which 

 we may account for the formation of types, acting, just as natural 

 selection does, in differentiating rather than in originating the 

 variation. No one can have followed the important and suggest- 

 ive works of Cope and Hyatt on the subject of acceleration and 

 retardation and not feel that it expresses an important law of this 

 kind. It is, as I understand it, a factor in evolution not compa- 

 rable with the principle of natural selection, but complementary 

 thereto, much in the same way as physiological selection and 

 sexual selection are. It is an attempt to give expression and form 

 to a set of facts to which palaeontology undoubtedly points and 

 which ontogeny substantiates, viz., that certain types may attain 

 perfection in time and then retrogress and finally become extinct, 

 and that existing types which are dying out, or degenerating, ex- 

 hibit, ontogenically, the culmination of force and complexity, fol- 

 lowed by decadence, corresponding to the phylogenic history of 

 the type. We know from the " Life and Letters " that Darwin 

 gave up in despair the attempt to grasp the full meaning of these 

 particular views of our associates, and, in a letter to Hyatt, with 

 characteristic modesty, he attributes this inability to his own 

 dullness rather than to any weakness in the theory. Others have 

 experienced the same difiiculty, and believe, with Prof. Morse, 

 that the facts enumerated, as well as the facts of exact and inex- 

 act parallelism, are explicable on the doctrine of natural selection. 

 This is true, it seems to me, only on the broader, unjustified inter- 

 pretation of the doctrine to which I have previously alluded in 

 the opening of these remarks. The law of acceleration and re- 

 tardation may, perhaps^ be substantially stated in this wise : that 

 certain groups acquire some characters rapidly, while correspond- 

 ing groups acquire the same characters more slowly, or never ac- 

 quire them at all ; and this brings us to another important factor 

 of evolution which serves to give force to the law. 



Acceleration by Primogeniture. — This has been elaborated by 

 Hubrecht. He argues that so long as the parent form remained 



