GROWTH AND ASEXUAL GENESIS. 423 



' — the breaking up of the parental mass into a number of 

 germs. An example is supplied by one of the lowest of the 

 class — the Gregarina. This creature, which is nothing more 

 than a minute spheroidal nucleated mass of protoplasm, 

 having a structureless outer layer denser than the rest, but 

 being without mouth or any organ, resolves itself into a 

 multitude of still more minute masses, which when set free 

 by bursting of the envelope, shortly become Amceha-iovm, 

 and severcilly assuming the structure of the parent, go 

 through the same course. Some of the Infusoria, as for in- 

 stance those of the genus Kolpoda, similarly become encysted 

 and subsequently break up into young ones. The 



more familiar mode of increase among these animal- aggre- 

 gates of the first order, by fission, though it sacrifices the 

 parent individuality by merging it in the individualities of 

 the two produced, sacrifices it less completely than does the 

 dissolution into a great number of germs. Occurring, how- 

 ever, as this fission does, very frequently, and being com- 

 pleted, in some cases that have been observed, in the course 

 of half-an-hour, it results in immensely- rapid multiplication. 

 If all its offspring survive, and continue dividing them- 

 selves, a single Paramecium 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." 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 ! 

 Making allowance for exagj. oration in these estimates, it is 

 beyond question that among these smallest of animals the 

 rate of asexual multiplication is by far the greatest ; and 

 this suiEces for the purposes of the argument.* 



* That these estimated rates are not greater than is probable, may be 

 inferred from such observations as that of Mr. Brightvvell on tlie buda 

 of Zoothamnium. "At nine in the morning, one of these buds, or Dva, was 



