44 INTRODUCTION. 



in the act of observing, but are also of the highest value to the 

 Physiologist, v?ho seeks to determine from the study of them 

 what are the acts v^herein Vitality may be said essentially to 

 consist, and what are the fundamental distinctions between Ani- 

 mal and Vegetable life. Thus it is among these plants, that we 

 can best study the history of the multiplication of cells by 

 " binary subdivision," which seems to be the most general mode 

 of growth and increase throughout the Vegetable kingdom _; and 

 it is in these, again, that the process of sexual generation is pre- 

 sented to us under its simplest aspect, in that curious act of 

 " conjugation" to which reference has already been made (p. 39). 

 But further, nearly all these Plants have at some period or other 

 of their lives, a power of spontaneous movement; which in many 

 instances so much resembles that of Animalcules, as to seem un- 

 mistakably to indicate their animal nature, more especially as 

 this movement is usuallj^ accomplished by the agency of visible 

 cilia : and the determination of the conditions under which it 

 occurs, and of the purposes it is intended to fulfil, is only likely 

 to be accomplished after a far more extensive as well as more 

 minute study of their entire history, than has yet been prose- 

 cuted, save in a small number of instances. It is not a little 

 remarkable, that in several of the cases in Avhich the life-history 

 of these plants has been rfiost completely elucidated, they have 

 been found to present a great variety of forms and aspects at 

 different periods of their existence, and also to possess several 

 different methods of reproduction ; and hence it can be very 

 little doubted, that numerous forms which are commonly reputed 

 to be distinct and unrelated species, will prove in the end to be 

 nothing else than successive stages of one and the same type. 

 One of the most curious results attained by jNIicroscopic inquiiy 

 of late years, has been the successive transfer of one group of 

 reputed Animalcules after another, from the Animal to the Vege- 

 table side of the line of demarcation between the two kingdoms ; 

 and although, as to the precise points across which this line 

 should be drawn, there is not yet a unanimous agreement, yet 

 there is now an increasing accordance as to its general situation, 

 which, even a few years since, was energetically canvassed. 

 Those who are acquainted with the well-known Volvox (com- 

 monly termed the " globe-animalcule ") will be surprised to learn 

 that this, with its allies, constituting the family Volvocinece, is 

 now to be considered as on the Vegetable side of the boundary. 

 (On the subjects of this paragraph, see Chap. VI.) 



I^ot only this lowest type of Vegetable existence, but the Cryp- 

 togamic series as a whole, has undergone of late years a very 

 close scrutiny, which has yielded results of the highest impor- 

 tance ; many new and curious forms having been brought to 

 light (some of them in situations in which their existence might 

 have been least anticipated), and some of the most obscure por- 

 tions of their history having received an unexpectedly clear eluci- 



