852 PPtOYISIONAL HYPOTHESIS Chap. XXVIL 



anotlier way ; for Trembley observed witli the hydra, that 

 the reproduction of the head after amputation was checked as 

 soon as the animal put forth reproductive gemmae.^ 



Between the production, by fissiparous generation, of two 

 or more complete individuals, and the repair of even a very 

 slight injury, there is so perfect a gradation, that it is 

 impossible to doubt that the two processes are connected. As 

 at each stage of growth an amputated part is replaced by one 

 in the same state of development, we must also follow Sir J. 

 Paget in admitting, " that the powers of development from the 

 " embryo, are identical with those exercised for the restora- 

 *' tion from injuries : in other words, that the powers are the 

 " same by which perfection is first achieved, and by which, 

 " when lost, it is recovered." ^ Finally, we may conclude 

 that the several forms of budding, fissiparous generation, the 

 repair of injuries, and development, are all essentially the 

 results of one and the same power. 



Sexual Generation. — The union of the two sexual elements 

 seems at first sight to make a broad distinction between 

 sexual and asexual generation. But the conjugation of 

 algae, by which process the contents of two cells unite into a 

 single mass capable of development, apparently gives us the 

 first step towards sexual union : and Pringsheim, in his 

 memoir on the pairing of Zoospores,"^ shows that conjugation 

 graduates into true sexual reproduction. Moreover, tlie now 

 well-ascertained cases of Parthenogenesis prove that the 

 distinction between sexual and asexual generation is not 

 nearly so great as was formerly thought ; for ova occasion- 

 ally, and even in some cases frequently, become developed into 

 perfect beings, without the concourse of the male. With 

 most of the lower animals and even with mammals, the ova 

 show a trace of parthenogenetic power, for without being 

 fertilised they pass through the first stages of segmentation 

 Nor can pseudova which do not need fertilisation, be dis- 



* Paget, ' Lectures on Pathology,' bold, " Ueber Parthenogenesis," ' Sit 

 1853, p. 158. zung der math. phys. Classe.' Munich, 



« Ibid., pp. 152, 164. Nov. 4th, 1871, p. 240. /SeealsoQua- 



^ Translated in ' Annais and Mag. trefages, 'Annales des Sc. Nat. Zoolog., 



of Nat. Hist.,' April, 1870, p. 272. 3rd Series, 1850, p. 138. 



* BischofF, as quoted by vou Sie- 





