94 THE BEGINNINGS OF LIFE. 



however long the developmental processes may be which 

 have to be passed through before the parent form can 

 be assumed. The human ovum develops as surely into 

 a human being, as that of a fish does into a similar fish. 

 The process is, in fact, everywhere the same — whether 

 we have to do with mammal, bird, reptile, or fish, or 

 whether the recently separated portion of an Amceba^ of 

 a Hydra^ or of some plant undergoes development. In 

 each case there is a reproduction of like from like, quite 

 irrespective of the grade of development which has been 

 attained by the parent organism. These phenomena 

 have been generalized under a ' Law of Heredity,' 

 >vhose meaning (after the most careful consideration 

 of the . facts) has been thus admirably rendered by 

 Mr. Herbert Spencer. He says ^ : — ^ Bringing the ques- 

 tion to its ultimate and simplest form, we may say 

 that, as on the one hand physiological units will, 

 because of their special polarities^ build themselves into 

 an organism of a special structure ; so, on the other 

 hand, if the structure of this organism is modified by 

 modified function, it ivill impress some corresponding 

 modification on the structure and polarities of its units. 

 The units and the aggregate must act and re-act on 

 each other. The forces exercised by each unit on the 

 aggregate and by the aggregate on each unit must 

 ever tend towards a balance. If nothing prevents, 

 the units will mould the aggregate into a form in 

 equilibrium with their pre-existing polarities. If, con- 



^ ' Principles of Biology,' vol. i. p. 256, 



