88 CRITIQUES AND ADDRESSES. [iv. 



chemical, in favour of that "immense unite de compo- 

 sition elementaire dans tous les corps vivants de la 

 nature," into which Payen had, so early, a clear insight. 

 As far back as 1850, Cohn wrote, apparently without 

 any knowledge of what Payen had said before him : 



" The protoplasm of the botanist, and the contractile substance and 

 saroode of the zoologist, must be, if not identical, yet in a high degree 

 analogous substances. Hence, from this point of view, the difference 

 between animals and plants consists in this ; that, in the latter, the con- 

 tractile substance, as a primordial utricle, is enclosed within an inert 

 cellulose membrane, which permits it only to exhibit an internal 

 motion, expressed by the phenomena of rotation and circulation, while, 

 in the former, it is not so enclosed. The protoplasm in the form of the 

 primordial utricle is, as it were, the animal element in the plant, but 

 which is imprisoned, and only becomes free in the animal ; or, to strip 

 off the metaphor which obscures simple thought, the energy of organic 

 vitality which is manifested in movement is especially exhibited by a 

 nitrogenous contractile substance, which in plants is limited and 

 fettered by an inert membrane, in animals not so." 1 



In 1868, thinking that an untechnical statement of 

 the views current among the leaders of biological science 

 might be interesting to the general public, I gave a 

 lecture embodying them in Edinburgh. Those who 

 have not made the mistake of attempting to approach 

 biology, either by the high a priori road of mere philo- 

 sophical speculation, or by the mere low d posteriori 

 lane offered by the tube of a microscope, but have taken 

 the trouble to become acquainted with well-ascertained 

 facts and with their history, will not need to be told 

 that in what I had to say " as regards protoplasm " in 

 my lecture " On the Physical Basis of Life/' there was 

 nothing new ; and, as I hope, nothing that the present 

 state of knowledge does not justify us in believing to 

 be true. Under these circumstances, my surprise may 

 be imagined, when I found, that the mere statement of 



1 Cohn, " Ueber Protococcus pluvialis," in the " Nova Acta" for 1850. 



