V] OF FORM AND SPECIES 413 



becomes more manifest from the point of view of physical and 

 mathematical analysis, and whose form is referable, or largely 

 referable, to the direct action of a physical force. When we come 

 to the minute skeletons of the Radiolaria we shall again find our- 

 selves dealing with endless modifications of form, in which it becomes 

 more and more difficult to discern, and at last vain and hopeless to 

 apply, the guiding principle of affihation or '^phylogeny.'' 



Among the Foraminifera we have an immense variety of forms, 

 which, in the hght of surface-tension and of the principle of minimal 

 area, are capable of explanation and of reduction to a small number 

 of characteristic types. Many of them are composite structures, 

 formed by the successive imposition of cell upon cell, and these we shall 

 deal with later on ; let us glance here at the simpler conformations 

 exhibited by the single chambered or " monothalamic " genera, and 

 perhaps one or two of the simplest composites. 



We begin with forms like Astrorhiza (Fig. 320, p. 703), which are 

 large, coarse and highly irregular, and end with others which are 

 minute and delicate, and which manifest a perfect and mathe- 

 matical regularity. The broad difference between these two types 

 is that the former are characterised, like Amoeba, by a variable 

 surface-tension, and consequently by unstable equihbrium; but the 

 strong contrast between these and the regular forms is bridged over 

 by various transition-stages, or differences of degree. Indeed, as 

 in all other Rhizopods, the very fact of the emission of pseudopodia, 

 which are especially characteristic of this group of animals, is 

 a sign of unstable surface-equihbrium ; and we must therefore 

 consider, or may at least suspect, that those forms whose shells 

 indicate the most perfect symmetry and equilibrium have secreted 

 these during periods when rest and uniformity of surface-conditions 

 contrasted with the phases of pseudopodial activity. The irregular 

 forms are in almost all cases arenaceous, that is to say they have 

 no soHd shells formed by steady adsorptive secretion, but only a 

 looser covering of sand grains with which the protoplasmic body 

 has come in contact and cohered. Sometimes, as in Ramulina, we 

 have a calcareous shell combined with irregularity of form; but 

 here we can easily see a partial and as it were a broken regularity, 

 the regular forms of sphere and cylinder being repeated in various 



