tlie^stoutest sliii^ that ever left dockyard would founder hopelessly ; 

 and contrast hhn with the invisible animalcules — mere gelatinous 

 specks, multitudes of which could, in fact, dance upon the point 

 of a needle with the same ease as the angels of the schoolmen 

 could, in imagmation. With these images before your minds, 

 you may well ask what community of form, or structure, is there 

 between the animalcule and the whale, or between the fungus and 

 the fig-tree? And, a fortiori^ between all four? 



Finally, if we regard substance, or material composition, 

 what hidden bond can connect the flower which a girl wears in 

 her hair and the blooc^^ych courses through her youthful veins ; 

 or, what is there ^^^^^^^^ between the dense and resisting 

 mass of the oak, o^^^^^^^g fabric of the tortoise, and those 

 broad disks of glasl^^^^^Kich may be seen pulsating through 

 the waters of a calm s^^HI Avhich drain away to mere films in 

 the hand which raises them out of their element ? Such objec- 

 tions as these must, I think, arise in the mind of every one who 

 ponders, for the first time, uj^on the conception of a single phys- 

 ical basis of life underlying all the diversities of vital existence ; 

 but I propose to demonstrate to you that, notwithstanding these 

 apparent difficulties, a threefold unity — namely, a unity of power 

 or faculty, a unity of form, and a unity of substantial composi- 

 tion — does pervade the whole living world. Ko very abstruse 

 argumentation is needed, in the first place, to prove that the 

 powers, or faculties, of all kinds of living matter, diverse as they 

 may be in degree, are substantially similar in kind. Goethe has 

 condensed a survey of all the powers of mankind into the well- 

 known epigram : 



" Warum treibt sicli das Volk so unci schreit ? Es will sicli ernitliron 

 Kinder zeugen, und sie nahren so gut es vermag. 



Weiter bringt es kein Mensch, stelV er sicli, Avie er audi will." 



In physiological language this means, that all the multifarious 

 and complicated activities of man are comprehensible under three 

 categories. Either they a^b immediately directed towards the 

 maintenance and development of the body, or they effect trans- 

 itory changes in the relative positions of parts of the body, or 

 they tend towards the continuance of the species. Even those 

 manifestations of intellect, of feeling, and of will, which Ave 

 riffhtlv name the hiirher faculties, are not excluded from tliis 



