34 ASEXUAL REPRODUCTION. [IXTROD. 



In the treatises on Comparative Anatomy which belong especi- 

 ally to the beginning of this century, the idea constantly recurs that 

 the series of segments of a metarnerically segmented form do in 

 some sort represent a series of individuals which have not detached 

 themselves from each other. Seen in the light of the Doctrine 

 of Descent this resemblance or analogy has been taken as a pos- 

 sible indication that the segmented forms may actually have had 

 some such phylogenetic history as this. By similar reasoning the 

 Metazoa have been spoken of as "Colonies" of Protozoa. Now 

 though we need not allow ourselves to be drawn away into these 

 and other barren speculations as to phylogeny, we may still note 

 the substance of fact which underlies them. For it is now 

 recognized that between the process by which the body of a iYa/,s 

 is metamerically segmented, and that by which it divides into a 

 chain of future " individuals," no line can be drawn : that the 

 process of budding, or of stabilization, by which one form gives 

 rise to a number of detached individuals, is often indistinguishable 

 from the process by which a near ally gives rise to a connected 

 colony, and that the two processes may even be interchangeable in 

 the same form ; finally that the process of division of a fertilized 

 ovum by the first cleavage plane may be in some essentials com- 

 parable with the division of a Protozoon into two new individuals. 

 All these are now commonplaces of Natural History. 



With what justice these considerations may have been applied 

 to the problems of phylogeny we need not now inquire, but to the 

 interpretation of the facts of Variation they have an application 

 which ought not to be neglected. 



If, then, as is admitted, there is a true analogy between the 

 process by which new organisms may arise asexually by Division, 

 and the process by which ordinary Meristic Series are produced, it 

 follows that Variation, in the sense of difference between offspring 

 and parent, should find an analogy in Differentiation between the 

 members of a Meristic Series. Applied to the case of asexual re- 

 production there seems no good reason for denying this analogy. 

 It is of course an undoubted fact that in the asexual reproduction 

 of many forms Variation is rare, though the sexually produced 

 offspring of the same forms are very variable. In plants this is 

 familiar to everyone, though the extension of the same principle 

 to animals rests chiefly on inference. Nevertheless in plants bud- 

 variation, both Meristic and Substantive, happens often, and the 

 division of a plant into two dissimilar branches may well be com- 

 pared to the production of dissimilar offspring by one parent. : in- 

 deed, if the processes of Division are admitted to be fundamentally 

 the same, this conclusion can scarcely be escaped. 



In one more aspect this subject may be considered with profit. 

 It is, as we have seen, believed that the division of an ovum into 

 two segmentation-spheres is not a process essentially different 



