446 SCIENTIFIC THOUGHT. 



biologists a formula which, like his physiological units, 

 has helped to give precision and direction to reasoning 

 on these subjects. But as growth has a natural limit 

 and leads to division, so reproduction through division 

 appears to have a limit also. " Only the very lowest 

 organisms, such as fission fungi, appear to be able to 

 multiply indefinitely by repeated divisions : for the 

 greater part of the animal and vegetable kingdoms the 

 general law may be laid down that, after a period of 

 increase of mass through cell division, a time arrives 

 45. when two cells of different origin must fuse together, 

 of two producing by their coalescence an elementary organism 



elements. 



which affords the starting-point for a new series of 

 multiphcations by division." ^ Fertilisation is now 

 known to be a cellular problem. As such it has been 

 studied in favourable cases which permitted of direct ob- 

 servation, and what has been ascertained in those cases 

 — exhibiting in general the same common features and 

 phases of development — has by inference under the great 

 generalisations of the cellular theory been extended to 

 all living things in which sexual differentiation exists, 

 be they animals or plants.^ The male and the female 



1 Hertwig, ' The Cell,' p. 2.r2. 

 The process may be looked at as an 

 instance of tlie cyclical order of 

 change. " The multiplication of the 

 elementary organism, anil with it 

 life itself, resolves itself into a 

 cyclic process. . . . Such cycles are 

 termed generation cycles. They 

 occur in the whole organic king- 

 dom in the most various forms." 

 Similarly Sir M. Foster (' Text-book 

 of Physiology,' 5th ed., p. l.^.")5), as 

 quoted, supra, p. 289. We may 

 add that from a still broader stand- 

 point, which we may call that of 



bionomics — in distinction from 

 biology — the cycle never repeats 

 itself, but, owing to overcrowding 

 and selection, something different, 

 more complex — i.e., externally or 

 internally better endowed — is pro- 

 duced. Philosophically we call this 

 progress. 



- There exists no more remark- 

 able instance of the extension of 

 natural knowledge by a process of 

 very incomplete induction than the 

 gradual linn establishment of the 

 now universally adopted doctrine of 

 fertilisation, no more brilliant refu- 



