194 Transactions of the Society. 



At the same moment a slit d appears at the posterior end, and 

 with this an incision in the nucleus. Both slits rapidly open ; two 

 anterior flagella ai"e formed, as at e ; the nucleus wholly divides, as 

 at/, and soon only a neck of sarcode connects the dividing halves, 

 as at g, h. Pulling in opposite directions then takes place, 

 lengthening the cord, as at i, until it reaches the fineness, and double 

 the length of the flagellum required for each divided form, as at k, 

 when it snaps and each is free. 



The mystery of all this simplicity of vital movement is deep ; 

 and although we can observe and in accurate manner record the 

 process, its modus operandi is far beyond our present grasp. 



One thing is certain, on this rapid power of self-multiphcation 

 depends the entire utility of these organisms, and in this function 

 of self-division it would appear that they have reached the highest 

 point of vital development. Lowly they are — we know of nothing 

 living that is lowlier — but in the processes of vital evolution, 

 amongst the lowly and simple as amongst the highly organized 

 and most complex, we find the perfection of concurrent adapta- 

 tion. 



One other feature in these minute organisms claims a note. 

 They are subject to no caprice ; after twelve years of close observa- 

 tion I am convinced that the vital processes are as orderly, rigid 

 and immutable as in the most complex organisms. Their life- 

 cycles are as clearly definable as those of a crustacean or a bird. 

 IS'o vital phenomenon not to be found amongst higher and larger 

 organisms, is discoverable here. Only the methods of specific 

 mutation resulting from the secular processes enunciated in the 

 Darwinian law are in operation. 



But the action of these processes is certain ; and from the 

 rapidity of their increase, the shortness of their life-histories, and the 

 ease with which vast multitudes may be studied, these organisms 

 afford a splendid field — by changes of environment — for proving the 

 action of the Darwinian law. But this must be a work in which it 

 would be vain to enter, unless, relatively speaking, vast time were 

 given. For, immense as their multitudes are, and rapidly as gene- 

 ration succeeds generation, and easily as changes of relative minute- 

 ness may be seen, it is manifest to a close observer that definite and 

 discoverable change, in response to a marked change of environment, 

 may be brought about; and there can be no doubt that the 

 tendency to vary is fully operative in the group; and with it a 

 tendency to conserve useful variation. This to an interesting ex- 

 tent I have proved, and shall hope on a future occasion to give you 

 the evidence afforded. 



IMeantime, however, I may remark, that so far as my observation 

 has extended it is never in the process of fission that variation 



