12 



time is necessary to produce an Independently existing specimen, 

 that the evohition may be said almost to be instantaneous. 



It sometimes happens that an Amoeba changing its form, as it 

 does every moment, shoots out a portion of its body to a greater 

 distance than usual ; sometimes the piece so projected will be in 

 the form of a finger, the end fm-thest ft-om the body vnll increase 

 in size, and the expanded end retain its position. The contraction 

 of the parent causes the connecting portion to become thinner and 

 thinner, until at last it is broken, and the continuity between 

 parent and offspring entirely ceases. 



Seeing that an Amoeba is alive, but is entirely without any 

 organ diiferentiated for a special purpose, it is probable that this 

 mode of reproduction is very commonly resorted to, at any rate it 

 is certain Amoeba are so produced ; and it is impossible to con- 

 ceive a more rapid production of an independent existence. 



The very short time required for Infusoria to increase from 

 units to millions is a fact well established ; one illustration will 

 suffice. 



A Paramecium, well supplied with food, undergoes fissiparous 

 division daily ; therefore 16,384 would be produced in a fortnight 

 and 268,435,456 in four weeks. 



I believe, however, there are species which reproduce even more 

 rapidly than the above. 



It must be noted, however, that the Paramecia not only mul- 

 tiply by spontaneous scission as stated, but also by genuine sexual 

 reproduction. Their multiplication by gamogenetic reproduction 

 is perhaps not quite so rapid as by agamogenetic ; the period of 

 gestation, if it can be so called, is usually from five to six days, 

 but of course several young are produced at a time. 



Turning to the higher Infusoria, the Rotatoria, we find the 

 time for reproduction very short. Ehrenberg wrote, that he 

 insulated a single specimen of Hydatina senta, and kept it in a 

 separate vessel for eighteen days ; that dm-ing this interval it laid 

 four eggs per diem, and that the young so produced, at two days 

 old, began to lay a like number. 



It can be easily seen that one million independent existences 

 would thus be produced in a few days. 



The Coelenterata stand next the Protozoa in an ascending scale 

 of organization. 



Many of the species of the Actinozoa, one of the two divisions 

 into which the Coelenterata are divided, reproduce by fissiparous 



