GROWTH AND ASEXUAL GENESIS. 443 



have been observed, in the course of half-an-hour, it results 

 in immensely- rapid multiplication. If all its offspring sur- 

 vive, and continue dividing themselves, a single Paramcecium 

 is said to be capable of thus originating 268 millions in the 

 course of a month.* Nor is this the greatest known rate 

 of increase. Another animalcule, visible only under a high 

 magnifying power, " is calculated to generate 170 billions 

 in four days." f And these enormous powers of propagation 

 are accompanied by a minuteness so extreme, that of some 

 species one drop of water would contain as many individuals 

 as there are human beings on the Earth ! Even if we allow 

 a large margin for exaggeration in these estimates, it is 

 beyond question that among these smallest of animals the 

 rate of asexual multiplication is immensely the greatest; 

 and this suffices for the purposes of argument. 



Of animal aggregates belonging to the second order, that 

 multiply asexually with rapidity, the familiar Polypes 

 furnish conspicuous examples. By gemmation in most cases, 

 in other cases by fission, and in some ases by both, the 

 agamogenesis is carried on among these tribes. As shown in 

 Fig. 148, the budding of young ones from the parent Hydra 

 is carried on so actively, that before the oldest of them is 

 cast off half-a-dozen or more others have reached various 

 stages of growth; and even while still attached, the first- 

 formed of the group have commenced budding out from their 

 sides a second generation of young ones. In the Hydra tuba 



* To meet a possible criticism it should bo remarked that this calcula- 

 tion assumes that the power of asexual reproduction is not exhausted by 

 the end of the month. It has been found that " the successive fissions of 

 Paramcecium cannot continue indefinitely. After some hundreds of genera- 

 tions the products of fission are small, have no mouth, and die unless before 

 this they have been allowed to conjugate with individuals of another brood." 

 It may, however, be fairly taken for granted that " some hundreds of genera- 

 tions " would take longer than a month. 



f Even this number is far exceeded. Dr. Edward Klein, in a lecture he 

 gave at the Royal Institution on June 2, 1898, asserted that 246 bacteria in 

 a cubic centimetre of nutritive liquid would multiply 'to 20,000,000 in the 

 course of twenty-four hours : a rate which, at the end of the third day, would 

 give, as the offspring of one individual, 537,367, 797,000,000. 



