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alike become stationary, and either dry up or encyst themselves. 
The cell-cycle in plants therefore is only found in its entirety in 
algae. Archegoniates are indeed terrestrial, hut their brief cell-cycle 
during fertilisation is absolutely dependent upon the abundant 
moisture, without access to which neither moss, liverwort, nor pro- 
thallium ever occurs. Thus it is that the higher terrestrial plants 
have become restricted to the encysted phase. Only to escape death, 
has the dryad become thus shut up within the tree ; but once so 
protected the extensive replacement of the cryptogams by the 
phanerogams, in all the less humid climates of the world, is readily 
accounted for. 
Passing to animal life, the vast preponderance of aquatic forms 
over terrestrial, is very similarly to be accounted for by the aid of 
the present theory. Only the higher members of a few groups have 
successfully emerged from their native element, and their existence 
depends upon their differentiation of an internal fluid medium, of that 
“ milieu int6rieur” upon which Claude Bernard was wont to lay such 
stress,* this in turn depending upon the early differentiation of in- 
ternal cavities. The interdependence of morphological and physio- 
logical theory will be sufflciently obvious from such considerations, f 
14. Theory of Variation . — The more completely one accepts and 
reflects upon the theory of natural selection, the more one feels the 
necessity for some view more satisfactory than heretofore of the 
causes of variation. J The present conception of the cell-cycle seems 
to go far towards supplying this. On the ordinary conception of 
the cell theory, that of the plant as an aggregate of encysted cells, 
and of the animal as an aggregate of essentially amoeboid ones, the 
organism cannot be credited with any innate variability — its ob- 
served variations are merely those which it receives, so to speak, 
between hammer and anvil — from the forces of the environment. 
The present conception, however, of all cells, however varied and 
specialised, being essentially differentiations from an encysted, an 
amoeboid or a ciliated form, and of these forms as phases of a 
single form-history, enables us to credit the cells and the resultant 
organism with an innate tendency to variation, and this along 
* Phenomenes de la vie communs aux an. et auxveg., Paris, 1879. 
t “Morphology,” Ency. Brit, ‘Eel. of Morphol. to Physiol.’ 
X Of. Origin of Species. 
