SECT, vn.] INTRODUCTION. 35 



from the division of certain Protozoa into two " individuals." In 

 conceiving the manner of Variation in such Protozoa we have 

 little or no fact to guide us, but this much is obvious : that for the 

 introduction of a variety as the offspring of a given species, it is 

 necessary either that the two parts into which the unicellular 

 organism divided should have varied equally, and that the 

 division should thus be a symmetrical division (in the full sense of 

 qualitative as well as formal symmetry) ; or that the division 

 should be asymmetrical, the resulting parts being dissimilar, in 

 which case one may conceivably belong to the type and the other 

 be a Variety. If Variation has ever occurred in the reproduction 

 of animals of this class it must have occurred on one or both of 

 these plans. 



Returning to the segmentation of the Metazoan ovum we have 

 the well-known results of Roux and others, shewing that, in 

 certain species, the first 1 cleavage-plane divides the body into the 

 future right and left halves. In such cases then on the analogy 

 of the Protozoon, the right and left halves of the body are in a 

 sense comparable with the two young Protozoa, and though each 

 half is hemi-symmetrical, it is in this way the equivalent of a 

 separate organism. This suggestion, which is an old one, receives 

 support from many facts of Meristic Variation, especially from the 

 mode of formation of homologous Twins and "double Monsters" 

 which are now shewn almost beyond doubt, to arise from the 

 division of one ovum 2 . But besides the evidence that each 

 half of the body may on occasion develop into a whole, evidence 

 will be given that one half may vary in its entirety, independently 

 of the other half. Such Variation may be one of sex, taking the 

 form of Gynandromorphy, so well-known among Lepidoptera, in 

 which the secondary sexual characters of one side are male, those 

 of the other being female ; or it may happen that the difference 

 between the two sides is one of size, the limbs and organs of one 

 side being smaller than those of the other ; or lastly the Variation 

 between the two sides may be one that has been held characteristic 

 of type and variety or even of so-called species and species 3 . 



These matters have been alluded to here as things which a 

 student of the facts of Variation will do well to bear in mind. It 

 is difficult to see the facts thus grouped without feeling the 



1 Often it is the second cleavage-plane (if any) which corresponds with the 

 future middle line. 



2 The well-known evidence relating to this subject will be spoken of later. The 

 view given above, which is now very generally received, finds support in the striking 

 observations of Dkiesch, lately published (Zt. f. w. Zool., 1891, liii. p. 160). 

 Working with eggs of Echimis, Driesch found that if the first two segmentation- 

 spheres were artificially separated, each grew into a separate Pluteus ; if the 

 separation was incomplete, the result was a double-monster, united by homologous 

 surfaces. Similar experiments attended by similar results have since been made on 

 Amphioxus by E. B. Wilson, Anat. Anz., vn. 1892, p. 732. 



a Evidence of such abrupt Variation between the two sides of the body belongs 

 for the most part to the Substantive group. 



3—2 



