THE INNER TISSUES OP ANIMALS. 363 



animals. If we contemplate the movements of cilia, of a 

 Rhizopod's pseudopodia, of a Polype's body, or of the long 

 pendant tentacles of a Medusa, we shall see great congruity 

 between them and this hypothesis. Bearing in mind that the 

 contractile substance of developed muscle is affected not by 

 nervous influence only, but, where nervous influence is de- 

 stroyed, is made to contract by mechanical disturbance and 

 chemical action, we may infer that it does not differ intrin- 

 sically from the primordial contractile substance which, in 

 the lowest animals, changes its bulk under other stimuli than 

 the nervous. We shall see significance in the fact ascer- 

 tained by Dr. Ransom, that various agents which excite 

 and arrest nervo-muscular movements in developed animals, 

 excite and arrest the protoplasmic movements in ova. We 

 shall understand how tissues not yet differentiated into mus- 

 cle and nerve, have this joint irritability and contractility; 

 how muscle and nerve may arise by the segregation of their 

 mingled colloids, the one of which, not appreciably altering 

 its bulk during isomeric change, readily propagates molecular 

 disturbance, while the other, contracting when isomerically 

 changed, less readily passes on the molecular disturbance; 

 and how, by this differentiation and integration of the con- 

 ducting and the contracting colloids, the one ramifying 

 through the other, it becomes possible for a whole mass to 

 contract suddenly, instead of contracting gradually, as it does 

 when undifferentiated. 



The question remaining to be asked is — What causes the 

 specialization of contractile substance? — What causes the 

 growth of colloid masses which monopolize this contractility, 

 and leave kindred colloids to monopolize other properties? 

 Has natural selection gradually localized and increased the 

 primordial muscular substance? or has the frequent recur- 

 rence of irritations and consequent contractions at particular 

 parts done it? We have, I think, reason to conclude that 

 direct equilibration rather than indirect equilibration has been 

 chiefly operative. The reasoning that was used in the case 



