THE FACTOES OP OEGANIC EVOLUTION. 53 



its persistent shape and accompanying greater rigidity of 

 the peripheral or ectoplasmic layer.&quot; * Describing generally 

 such low forms, some of which are said to have neither 

 nucleus nor vacuole, he remarks that in types somewhat 

 higher &quot; the outer or peripheral border of the protoplasmic 

 mass, while not assuming the character of a distinct cell- 

 wall or so-called cuticle, presents, as compared with the 

 inner substance of that mass, a slightly more solid type of 

 composition.&quot; t And it is added that these forms having so 

 slightly differentiated an exterior, &quot; while usually exhibiting 

 a more or less characteristic normal outline, can revert at 

 will to a pseud-amoeboid and repent state.&quot; J Here, then, 

 we have several indications of the truth that the permanent 

 externality of a certain part of the substance, is followed 

 by transformation of it into a coating unlike the substance 

 it contains. Indefinite and structureless in the simplest of 

 these forms, as instance again the Gregarina, the limiting 

 membrane becomes, in higher Infusoria, definite and often 

 complex: showing that the selection of favourable varia 

 tions has had largely to do with its formation. In such 

 types as the Foraminifera, which, almost structureless 

 internally though they are, secrete calcareous shells, it is 

 clear that the nature of this outer layer is determined by 

 inherited constitution. But recognition of this consists 

 with the belief that the action of the medium initiated the 

 outer layer, specialized though it now is ; and that even 

 still, contact with the medium excites secretion of it. 



A remarkable analogy remains to be named. When 

 we study the action of the medium in an inorganic mass, 

 we are led to see that between the outer changed layer 

 and the inner unchanged mass, comes a surface where 

 active change is going on. Here we have to note that, alike 

 in the plant-cell and in the animal-cell, there is a similar 

 relation of parts. Immediately inside the envelope comes 



* 16. Vol. i, p. 241. f Kent, Vol. i, p. 56. i Ib. Vol. i, p. 57. 



The Elements of Comparative Anatomy, by T. H. Huxley, pp. 7-9. 



