C. — GEOLOGY, }fl 



analysis; that no palaeontologist can guarantee the genetic purity of the 

 assemblages with which he works, even when his specimens are collected 

 from a single locality and horizon. It is difficult to reply to such 

 negative arguments. One can but give examples of the kind of obser- 

 vafion on which palaeontologists rely. 



Since Dr. Rowe's elaborate analysis of the species of Micraster 

 occurring in the Chalk of S.E. England, much attention has been 

 concentrated on the gradual changes undergone by those sea-urchins 

 in the course of ages. The changes observed affect many characters; 

 indeed, they affect the whole test, and all parts are doubtless correlated. 

 The changes come in regularly and gradually; there is no sign of 

 discontinuity. It is convenient to give names to the successive forms, 

 but they are linked up by innumerable gradations. There does not 

 seem here to be any question of the sudden appearance of a new 

 character, in one or in many individuals ; or of the introduction of 

 any character and the gradual extension of its range by cross-breeding 

 until it has become universal and in turn gives way to some new 

 step in advance. The whole assemblage is affected and moves forward 

 in line, not with an advanced scout here and a straggler there. Slight 

 variation between contemporaneous individuals occurs, no doubt, but 

 the •limits are such that a trained collector can tell from a single fossil 

 the level at which it has been found. The continuity of the changes is 

 also inferred from such a fact as that in occasional specimens of 

 Micraster cor-bovis the distinctness of the ambulacral sutures (which 

 is one of these characters) is greater on one side of the test than on 

 the other. 



Such changes as these may profitably be compared with those which 

 Professor Duerden believes to be taking place in the ostrich. He too 

 finds a slow continuous change affecting innumerable parts of the bird, 

 a change that is universal and within slight limits of variation as 

 between individuals. Even on the hypothesis that every barb of every 

 feather is represented by a factor in the germ, he finds it impossible 

 to regard the changes as other than continuous, and he is driven to 

 the supposition (on the hypothesis of germinal factors) that the factors 

 themselves undergo a gradual change, which he regards as due to old 

 age. It is interesting also that he finds an occasional lop-sided change, 

 such as we noted in Micraster cor-bovis. 



Whatever may be the explanation, the facts do seem to warrant 

 the statement that evolutionary change can be, and often is, continuous. 

 Professor De Vries has unfortunately robbed palaeontologists of the 

 word ' mutation,' by which, following Waagen, they were accustomed 

 to denote such change. I propose, therefore, to speak of it as 

 'transition. ' But here the question may be posed, whether such transi- 

 tions can progress indefinitely, or whether iliey should not be compared 

 to those divergences from the norm of a species which we call fluctua- 

 tions, because, like the waves of the sea piled up by a gale, they return 

 to their original level when the external cause is removed. If every 

 apparent transition in time is of the latter nature, then, when it reaches 

 a limit comparable to that circumscribing contemporary fluctuation, 

 there must, if progress is to persist, be some disturbance provoking 



