488 THE AMERICAN NATURALIST [Vol. LYI 



was compelled to assume under the influence of surface- 

 tension, underwent an important modification, the effect 

 of which has impressed itself upon all later develop- 

 ments. A spherical organism floating in the water and 

 growing under the direct influence of light would ob- 

 viously grow more rapidly on the upper side, where the 

 light first strikes it, than it would on the lower side away 

 from the light. There followed, therefore, an elongation 

 of the sphere in the vertical direction, and the definite 

 establishment of an anterior end, the upper end which 

 lay towards the light and at which the most vigorous 

 growth took place. In this way there was established a 

 definite polarity, which has persisted in all higher organ- 

 isms, a distinction between an anterior and a posterior 

 end. With the concentration of organic substance which 

 took the form of nucleus and reserve food supply, the 

 specific gravity of the plasma would become greater than 

 that of the surrounding water and the organism would 

 tend to sink. The necessity, therefore, arose for some 

 means of keeping it near the surface, that it might con- 

 tinue to grow under the influence of light. The response 

 to this need, however it was attained, came in the de- 

 velopment of an anterior flagellum. This we may regard 

 as an elongation in the direction of the light of a contrac- 

 tile portion of the external layer, moving rhythmically, 

 which by its movement counteracted the action of gravity, 

 and acting as a tractor drew the primitive flagellate up- 

 wards towards the surface layers, into a position where 

 further growth was possible. That this speculation of 

 Church's represents what was actually accomplished, 

 even though it does not make clear the means by which 

 it was brought about, is shown by the interesting re- 

 searches of Wager" on the rise and fall of the more highly 

 organized flagellate Euglena. Euglena is a somewhat 

 pear-shaped flagellate, the tapering end being anterior 

 and provided with a single flagellum, which acts as a 

 tractor drawing the organism towards the light. The 



sP/ji'. Trans. Boy. Soc, Vol. 201, 1911; an.l Scirnce Progress, Vol. vi. 

 October, 1911, p. 298. 



