52 THE FACTOES OP ORGANIC EVOLUTION. 



results from transformation of it, matters not to the argu 

 ment. Either way the action of the medium causes its 

 formation ; and either way the many varied and complex 

 differentiations which developed cell-walls display, must be 

 considered as originating from those variations of this 

 physically-generated covering which natural selection has 

 taken advantage of. 



The contained protoplasm of a vegetal cell, which has 

 self - mobility and when liberated sometimes performs 

 amoeba-like motions for a time, may be regarded as an 

 imprisoned amoeba ; and when we pass from it to a free 

 amoeba, which is one of the simplest types of first animals, 

 or Protozoa, we naturally meet with kindred phenomena. 

 The general trait which here concerns us, is that while 

 its plastic or semi-fluid sarcode goes on protruding, in 

 irregular ways, now this and now that part of its peri 

 phery, and again withdrawing into its interior first one 

 and then another of these temporary processes, perhaps 

 with some small portion of food attached, there is but 

 an indistinct differentiation of outer from inner (a fact 

 shown by the frequent coalescence of the pseudopodia in 

 Rhizopods) ; but that when it eventually becomes quiescent, 

 the surface becomes differentiated from the contents : the 

 passing into an encysted state, doubtless in large measure 

 due to inherited proclivity, being furthered, and having 

 probably been once initiated, &quot;fyy the action of the medium. 

 The connexion between constancy of relative position among 

 the parts of the sarcode, and the rise of a contrast between 

 superficial and central parts, is perhaps best shown in the 

 minutest and simplest Infusoria, the Mojiadlnse. The genus 

 Monas is described by Kent as &quot;plastic and unstable in form, 

 possessing no distinct cuticular investment ; . . . the food- 

 substances incepted at all parts of the periphery &quot;;* and 

 the genus Scytomonas he says &quot; differs from Monas only in 



* A Manual of the Infusoria, by W. Saville Kent. Vol. i, p. 232. 



