536 SPONTANEOUS GEMMATION. 



ally ? The manner of extension of any given plant is by bud after bud 

 in succession, either terminal or axillary ; but this extension does not go 

 on indefinitely ; it reaches a limit both as respects size and duration. 

 We never notice in the development of a bud which remains attached to 

 its parent stock the spontaneous appearance of novel qualities. The 

 flowers and fruits are like all the others upon the same plant. If such 

 a bud, then, removed from its parent seat, be permitted, under favorable 

 conditions, to grow elsewhere, it might be expected, as is actually the case, 

 that it would go on in its development without exhibiting any alterations. 



Essentially of an exhausting nature, reproduction by gemmation is 

 limited. It can only be repeated a definite number of times. At the 

 most, all that we do in this artificial process is to obtain a part of an old 

 individual under a new and isolated form. We thereby relieve such 

 new growth from the chance of those accidents which may befall the 

 original stock ; but both for the one and for the other there is a definite 

 term of life. When that term is approached, though we may take sci- 

 ons or buds, and treat them with every care in the usual operation of 

 grafting or budding, the operation will fail. 



There is a certain analogy between this incorporation of the parts of 

 different plants and the so-called grafting or Taliacotian operations which 

 are sometimes performed on the parts of animals, as the transplantation 

 of the spur of one bird on the top of the comb of another, or many of the 

 plastic operations of surgery ; but these parts do not necessarily perish 

 in the manner which has been indicated by Butler in his Hudibras. 



Propagation by gemmation and reproduction by generation are, in 

 many instances in the animal series, resorted to alternately for the con- 

 tinuation of the race. Thus, during the summer season, propagation by 

 gemmation may serve to increase the number of a given kind, but if these 

 should be unable to maintain themselves during the cold of winter, the 

 race would inevitably become extinct, unless reproduction by ova were 

 resorted to ; for though the developed animal may not be able to with- 

 Influence of stand the decline of temperature, the ova may. Thus, in 

 spontaneous 011 a hydra, propagation by gemmation continues until the ex- 

 gemmation, ternal temperature lowers to a certain degree, and that at 

 once brings on a reversion to the other process. The same thing has 

 been observed in the case of the aphis, which multiplies by gemmation 

 until there is a reduction of temperature, and then it multiplies by gener- 

 ation. We have already dwelt at length on the control which external 

 circumstances have over development; it is, therefore, no more than might 

 be expected that they should, in like manner, determine the processes of 

 propagation and reproduction. 



Gemmation occurs only in a very doubtful way and under special cir- 

 cumstances among the more advanced members of the animal series. In 



