GREATER PROBLEMS OF 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.’ 
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 
1 Cf. Giard, “Discours inaugurale,’”’” Bull. Scientif., iii, p. 1, 1888. 
