GREATER PROBLEMS OP BIOLOGY — THOMPSON. 389 



case where the resultant form can be well explained by, and can not 

 possibly be understood without, the phenomena of surface tension. 

 Indeed, in many of the simpler cases, the facts are so well explained 

 by surface tension that it is difficult to find place for a conflicting, 

 much less an overriding force. 



I believe, for my own part, that even the beautiful and varied 

 forms of the foraminifera may be ascribed to the same cause, but 

 here the problem is a little more complex, by reason of the successive 

 consolidations of the shell. Suppose the first cell or chamber to be 

 formed, assuming its globular shape in obedience to our law, and 

 then to secrete its calcareous envelope. The new growing bud of 

 protoplasm, accumulating outside the shell, will, in strict accordance 

 with the surface tensions concerned, either fail to "wet" or to adhere 

 to the first-formed shell, and will so detach itself as a unicellular indi- 

 vidual (Orbulina) ; or else it will flow over a less or greater part of 

 the original shell, until its free surface meets it at the required angle 

 of equilibrium. Then, according to this angle, the second chamber 

 may happen to be all but detached (Globigerina) , or, with all inter- 

 mediate degrees, may very nearly wholly enwrap the first. Take 

 any specific angle of contact, and presume the same conditions to 

 be maintained, and therefore the same angle to be repeated as each 

 successive chamber follows on the one before; and you will thereby 

 build up regular forms, spiral or alternate, that correspond with 

 marvelous accuracy to the actual forms of the foraminifera. And 

 this case is all the more interesting, because the allied and successive 

 forms so obtained differ only in degree, in the magnitude of a single 

 physical or mathematical factor; in other words, we get not only 

 individual phenomena, but lines of apparent orthogenesis, that seem 

 explicable by physical laws, and attributable to the continuity 

 between successive states in the continuous or gradual variations of 

 a physical condition. The resemblance between allied and related 

 forms, as Hartmann demonstrated, and Giard admitted years ago, 

 is not always, however often, to be explained by common descent 

 and parentage. 1 



In the segmenting egg we have the simpler phenomenon of a 

 laminar system, uncomplicated by the presence of a solid framework; 

 and here, in the earliest stages of segmentation, it is easy to see the 

 correspondence of the planes of division with what the laws of surface 

 tension demand. For instance, it is not the case (though the ele- 

 mentary books often represent it so) that when the totally segment- 

 ing egg has divided into four segments, these ever remain in contact 

 at a single point ; the arrangement would be unstable, and the position 

 untenable. But the laws of surface tension are at once seen to be 



« Cf. Giard, "Discours inaugurate," Bull. Scientif., iii, p. 1, 1888. 



