GENESIS. 235 



horaogenesis and lieterogenesis to the needs of the different 

 classes of organisms which exhibit them. 



One of the facts to be observed, is, that heterogenesis pre- 

 vails among organisms of which the food, though abundant 

 compared with their expenditure, is dispersed in such a way 

 that it cannot be appropriated in a wholesale manner. Pro- 

 iophyta, subsisting on diffused gases and decaying organic 

 matter in a state of minute subdivision ; and Protozoa, to 

 which food comes in the shape of extremely small floating 

 particles ; are enabled by their rapid agamogenetic multipli- 

 cation, to obtain materials for growth, better than they would 

 do did they not thus continually divide and disperse in pur- 

 suit of it. The higher plants, having for nutriment the car- 

 bonic acid of the air and certain mineral components of the 

 soil, show us modes of multiplication adapted to the fullest 

 utilization of these substances. A herb, with but little power 

 of forming the woody-fibre requisite to make a stem that can 

 support wide-spreading branches, after producing a few sex- 

 less axes, produces sexual ones ; and maintains its race better 

 by the consequent early dispersion of seeds, than by a further 

 production of sexless axes. But a tree, able to lift its suc- 

 cessive generations of sexless axes high into the air, where 

 each axis gets carbonic acid and light almost as freely as if it 

 grew by itself, may with advantage go on budding-out sex- 

 less axes year after year; since it thereby increases its sub- 

 sequent power of budding-out sexual axes. Meanwhile, it 

 may advantageouslj^ transform into seed-bearers, those axes 

 which, in consequence of their less direct access to materials 

 absorbed by the roots, are failing in their nutrition ; for iu 

 doing this, it is throwing-off from a point at which sus- 

 tenance is deficient, a migrating group of germs that may 

 find sustenance elsewhere. The heterogenesis displayed by 

 animals of the Cojlenterate type, has evidently a like utility. 

 A polype, feeding on minute annelids and crustaceans, which, 

 flitting through the water, come in contact with its tentacles; 



