REPRODUCTION 23 



with that of " chemiotaxy " for substances that in weak solution 

 are attractive or repellent to the being. Paramecium, which feeds 

 on bacteria, organisms of putrefaction, is positively chemiotactic 

 to solutions of carbon dioxide, and as it gives this off in its own 

 respiration, it is attracted to its fellows. The special case of 

 reaction to gases in solution is termed " aerotaxy," or " pneumo- 

 taxy," according as the gas is oxygen or carbon dioxide. We 

 find that in this respect there are degrees, so that a mixed 

 culture of Flagellates in an organic infusion sorts itself out, 

 under the cover of a microscopic preparation, into zones of 

 distinct species, at different distances from the freely aerated 

 edge, according to the demands of each species for oxygen and 

 CO2 respectively. 



Finally, we must note that the apparently "spontaneous 

 movements " of Protists can hardly be explained as other than 

 due either to external stimuli, such as we have just studied, or to 

 internal stimuli, the outcome of internal changes, such as fatigue, 

 hunger, and the lil^e. Of the latter kind are the movements that 

 result in eepeoduction. 



Reproduction. — We have noted above that the growth of an 

 organism which retains its shape alters the ratio of the surface 

 area to the whole volume, so necessary for the changes involved 

 in life. For the volume of an organism varies as the cube of 

 any given diameter, whereas the surface varies with the square 

 only. Without going into the arithmetical details, we may say 

 that the ratio of surface to volume is lessened to roughly four-fifths 

 of the original ratio when the cell doubles its bulk. As 

 Herbert Spencer and others have pointed out, this must reduce 

 the activities of the cell, and the due ratio is restored by the 

 division of the cell into two.^ This accounts for what we must 

 look on as the most primitive mode of reproduction, as it is the 

 simplest, and which we teim " fission " at Spencer's " limit of 



^ Let us take the case of a l-centimetre cube, growing to the size of a 2-ceiitimetre 

 cube. The superficial area of the 1 cm. cube measures 6 square centimetres, and 

 its bulk is 1 cubic centimetre. The superficial area of the 2-centimetre cube 

 measures 24 square centimetres, while its volume measures 8 cubic centimetres. 

 Tlius the larger cube has only 3 em. sq. of surface to every cubic cm. of volume, 

 instead of 6 ; in other words, the ratio of surface to volume has been halved by 

 growth. Three successive bipartitions of the larger cube will divide it into eight 

 separate 1-centimetre cubes, each now. possessing the original ratio of surface to 

 volume. 



