XII]' OF THE FORAMINIFERA 867 



on round and round the system, so building up a circular flattened 

 disc. This again we perceive to be, mathematically, a hmiting case 

 of the logarithmic spiral; the spiral has become wellnigh a circle and 

 the constant angle is wellnigh 90°. 



Lastly there are a certain number of Forarninifera in which, 

 without more ado, we may simply say that the arrangement of the 

 chambers is irregular, neither the law of constant ratio of magnitude 

 nor that of constant form being obeyed. The chambers are heaped 

 pell-mell upon one another, and such forms are known to naturahsts 

 as the Acervularidae. 



A B 



Fig. 431. A, Textularia trochus d'Orb. B, T. concava Karrer. 



While in these last we have an extreme lack of regularity, we 

 must not exaggerate the regularity or constancy which the more 

 ordinary forms display. We may think it hard to believe that the 

 simple causes, or simple laws, which we have described should operate, 

 and operate again and again, in millions of individuals to produce 

 the same delicate and complex conformations. But we are taking 

 a good deal for granted if we assert that they do so, and in particular 

 we are assuming, with very little proof, the "constancy of species" 

 in this group of animals. Just as Verworn has shewn that the 

 typical Amoeba proteus, when a trace of alkali is added to the water 

 in which it lives, tends, by alteration of surface tensions, to protrude 

 the more delicate pseudopodia characteristic of A. radiosa — and 

 again when the water is rendered a little more alkaline, to turn 

 apparently into the so-called A. liTnax — so it is evident that a very 

 slight modification in the surface-energies concerned might tend 



