78 Royal Institution : — 



the distinct apertures coalesce into one large jagged orifice, the 

 borders of which become more and more deeply cut, until they 

 present the ramifying extensions characteristic of Dendritina. Now, 

 if, in such a series, we once begin to make a distinct species for 

 every well-marked dissimilarity, either in the form of the shell, or 

 in that of the aperture, we must multiply our species almost in- 

 definitely, contrary to all probability; and there is no medium 

 between doing this, and uniting the whole series of forms included 

 in these two reputed genera under one specific type. This is the 

 more remarkable, because in one locality we may find only the 

 Dendritina-form, in another only the Peneroplis-form, whilst the 

 transitional or intermediate forms come from a third. 



Another remarkable example of this wide range of specific 

 characters is presented in the Orbitolite, a composite organism, 

 which, originating in a spheroidal nucleus of sarcode, increases by 

 the formation of new segments in concentric rings around this, so 

 that, each segment becoming invested with a shelly envelope, a very 

 beautiful disk is formed, which is enlarged by successive additions 

 to its margin. The segments communicate with each other by 

 annular canals ; and there are also passages connecting each annulus 

 with those within and without ; whilst from the outermost annulus 

 there are passages opening at the margin of the shelly disk, through 

 which alone the pseudopodia issue that obtain the food for the 

 whole organism. Now there are two very distinct types of growth 

 presented by these Orbitolites : one, namely, in which the disk is 

 very thin, and the segments form (as it were) but a single floor ; 

 and the other in which the disk becomes comparatively thick 

 through the vertical elongation of the segments, which, moreover, 

 are themselves partially divided into at least three distinct stories ; 

 two, namely, which form the two surfaces of the disk, and an inter- 

 mediate one, which is very distinctly separated from them both. 

 The former type of growth may be designated as the simple, the 

 latter as the complex, Now some Orbitolites seem to go through 

 their whole lives upon the simple plan, whilst in others the complex 

 plan shows itself in the very first ring ; and from the comparison 

 of such alone, it might be fairly supposed that these two plans are 

 characteristic of two distinct species. But when a considerable 

 number of these forms are examined, it appears that the simple 

 type may pass into the complex at any period of its growth ; the 

 same disk presenting the simple plan in the first 5, 10, 20, 30, or 

 more annuli, and the complex in all those subsequently formed. 

 Hence there can be no question that even so marked a diversity in 

 plan of growth is not in that case sufficient to establish a diversity 

 of specific type, but that the two must be accounted varieties only. 



A no less remarkable range of variation has been shown by 

 Professor Williamson and Mr. W. K. Parker to prevail in other 

 groups of Foraminifera which they have particularly studied ; so 

 that it would appear as if this type of animal existence were 

 specially characterized by its tendency to such variations. And 

 this will seem the more probable, when it is considered how little of 



