Heuon-Allkn and Eakland — Stuthj oj Vernewlina polijslroplin. lo/i 



In studying the evolution of species we must, as Frederick Chapman has 

 pointed out, make an arrangement taking the form of a net, in whicli tlie 

 species are represented by tlio knots which unite the threads, the threads 

 standing for the series of intermediate forms connecting the species/' Such 

 a pUxn was adopted in our Clare IsUmd Monograph,' in an attempt to group 

 the salient species and intermediate forms of the genus Discorbina. 



Eeturning now to our triangle theory, we postulate that every group of 

 organisms may be graphically represented by a series of triangles, the three 

 sides of which represent respectively Varieties, Species, Genera; 



and these triangles may find themselves juxtaposed in any way. The 

 juxtaposed faces of the triangle would be connected by x:cx, representing 

 " sports,'' or intermediate specimens, which might eventually take the form of 

 epidemic varieties linking one species, genus, or variety with another species, 

 genus, or variety; and such intermediate specimens " .r," would vary both 

 indefinitely and infinitely. 



Let us give another homely illustration of what appears to us to take place 

 in the evolution of genera and species. At all stages it would appear that 

 evolution may be illustrated by a hollow sphere (or pyramid, if we would 

 pursue the triangle theory), the sides of which are formed of a network. 

 Within this hollow figure we put a freely moving ball, appro .ximately the 

 same size as the meshes of the network. The ball represents the species ; the 

 network the accepted limits of variation within specific range. If the moving 

 ball sticks in the network, it becomes an established " variety" ; but if, under 

 some biological or local impulse or stimulus, the ball forces its way through 

 the meshes, it will not return, but will have evolved into a new "species " or 

 " genus," and will thenceforth move freely within its own new cage (or triangle 

 or pyramid). It will, in fact, have established its own triangle, and settled 

 into its place juxtaposed to its nearest ally, which is not necessarily the parent 

 form from which it derived its origin. 



'^ F. Chapman : " The Foramiuifera." London, 1902, p. 55. 



" E. Heron-Allen and A. Earland : " The Foraniinifera of the Clare Island District." 

 Clare Island Survey, Pt. (U. Pioc. R. Irish Acad., vol. x.\xi. Dublin, 1013, p. G4 

 (Table). 



Lr2] 



