Scientific Lectures. 25 



There are forms still more minute than these. Of the N^aviGula 

 radiosa., one lumdred and fifty thousand millions ; and of the Navi- 

 cula Mediterranean three hundred and fifty thousand millions could 

 be contained in the same small compass. The JS'avicula vlridis is 

 the smallest of the vegetable forms of which I have the measure- 

 ment ; and this is so minute that a cubic inch would easily contain 

 more than a million of millions. And the minuter monad forms of 

 which mention was just now made, are smaller than these in the 

 ratio of two to one. 



Whether the lowest limit of organic life has yet been reached by 

 the power of the microscope, is, perhaps, uncertain. The mona>s C7'e- 

 jpuscidion.^ the twilight monad, was so named by Ehrenberg as mark- 

 ing, in his opinion, the dawn of life ; but though among monads 

 specific differences are not very sharply defined, there are monads 

 now in the catalogue which do not measure in diameter more than 

 the tenth part of the measurement given by him for the tnonas cre- 

 piisculum. 



Upon the process of the development and growth of living things, 

 and of the building up of the most complicated structures from the 

 most simple germs, the microscope has thrown a light which could 

 have been derived from no other source. Thus, the multiplication 

 of cells by binary subdivision, which seems to be a universal law, is 

 a discovery exclusively due to the microscope; and this instrument 

 furnishes us all the knowledge we possess of those acts which are 

 essentially distinctive of vitality. From this law of binary subdi- 

 vision there follows a curious consequence in regard to those beings 

 which consist in tlieir full development of only a single cell. For, 

 whereas, by cell subdivisions the more complicated organisms grow 

 larger, but maintain, after all, a compound individuality still, among 

 the desmids and diatoms every subdivision of a cell produces two 

 distinct individuals where tliere was only one before. And what is 

 true of the unicellular plants is equally true of the unicellular ani- 

 mals. Every subdivision of a single animal — and this process of 

 subdivision is one which may be watched with the microscope from 

 beginning to end — produces two animals in all respects perfectly 

 similar, of which it would be impossible to say that either the one or 

 the other is parent or offspring. This process among these humble 

 forms of life is going on incessantly. As a consequence, if there be 

 but one or two individuals in an infusion to-day, there may be thou- 

 sands to-morrow. Prof. Ehrenberg, from his observations upon the 



