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 

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



Fusioii . 



of two producing by their coalescence an elementary organism 



elements. 



which affords the starting-point for a new series of 

 multiplications by division." ] Fertilisation is now 

 known to be a cellular problem. As such it lias 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. 2 The male and the female 



i Hertwig, ' The Cell,' p. 252. 

 The process may be looked at as an 

 instance of the cyclical order of 

 change. " The multiplication of the 

 elementary organism, and 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. 1555), 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. 



2 There exists no more remark- 

 able instance of the extension of 

 natural knowledge by a process of 

 very incomplete induction than the 

 gradual firm establishment of the 

 now universally adopted doctrine of 

 fertilisation, no more brilliant refu- 



