XI.] PROBLEMS OF THE DAY. 77 



means of the distinction between somatic and germ-cells, 

 natural selection was enabled to direct its attention, to speak 

 metaphorically, to the immortality of the germ-cells, and to 

 an entirely different range of properties among the somatic 

 cells, such as the capacity for movement, irritability, increased 

 powers of assimilation, &c. &c. We do not yet know whether 

 an increase in these properties is directly connected with a 

 change of constitution involving the loss of immortality, but it is 

 not impossible that this maybe the case ; and, if so, the somatic 

 cells would have ceased to be immortal more quickly than if 

 panmixia were the only agency at work. 



I have adduced in my fourth essay ^ the cases of the Volvo- 

 cinean genera, Volvox and Pandorina, as examples of the 

 differentiation of the lowest heteroplastids from the homo- 

 plastids. All the cells of Pandorina are similar and perform 

 similar functions. Volvox, on the other hand, consists of 

 somatic and germ-cells, and it is here that we should expect 

 the introduction of natural death. Dr. Klein's recent observa- 

 tions ^ show that this, as a matter of fact, takes place : as soon 

 as the germ-cells are matured, and have left the body of the 

 Alga, the flagellate somatic cells begin to shrink, and in one or 

 two days are all dead. This is all the more interesting because 

 the somatic cells fulfil nutritive functions for the aggregate. 

 It is true that they are not alone in performing the office of 

 assimilation, for the germ- cells also contain chlorophjdl ; but 

 the immense size which the latter attain in Volvox can only be 

 explained on the supposition that they receive nutriment from 

 the somatic cells. These cells are so constituted that they 

 assimilate, but when once the spherical colony has attained its 

 definite size they have ceased to grow. By means of a fine 

 protoplasmic network the body-cells pass on to the germ-cells 

 all the nutriment they acquire from the decomposition of 

 carbon dioxide and water, and when the reproductive cells are 

 mature they die. In this case adaptation for suppl3'ing nutri- 

 ment to the germ-cells may have hastened the introduction of 

 death among the somatic cells, inasmuch as some structure ma}'' 



^ See Vol. I, p. 163. 



^ Ludwig Klein, ' Morphologische und Biologische Studien uber die 

 Gattung Volvox.' Pringsheim's Jahrbucher fur wissenschaftliche Botanik, 

 Bd. XX. 1889. 



