THEORIES OF DEVELOPMENT 257 



things. Analogies can be found, perhaps, in inorganic phenomena, 

 as for instance a storm dividing into two or more parts and each 

 developing a new storm centre of its own, or when a suspended drop 

 is divided and each half becomes a new sphere ; but these comparisons 

 lack some of the essential features of the organic phenomenon. 



A progressive change takes place as development proceeds, so that 

 a stage once passed through is not repeated if a part is separated from 

 the rest, as illustrated by Driesch's experiments with the blastula and 

 gastrula of the sea-urchin and starfish, and by the method of develop- 

 ment of pieces of the adult, that do not pass through the embryonic 

 stages. As the protoplasm changes new conditions may arise, either 

 because the protoplasm in its new form can be acted upon by -those 

 internal or external conditions to which it did not respond at first, as 

 Driesch has supposed, or, as I think equally probable, because the 

 series of reactions that have begun with the first step in the develop- 

 ment work themselves out in the same way that a chemical reaction 

 once started may pass through a long series of stages depending upon 

 the nature of the substance. The difference between these views lies 

 in this, that the former supposes latent substances, or elementary 

 processes or forces, whatever they may be called, to be present in the 

 egg and to act when a medium that responds to them has come into 

 existence; the other idea supposes that the whole process is started with 

 the first change and once set going is of such a kind as to continue 

 to an end through a regular series of stages. Both views are suppo- 

 sitions, and, it may be, reduce themselves ultimately to the same thing. 



On any theory of development, the nucleus cannot be left out of 

 account, since the evidence that we now possess shows that through 

 the nucleus even the most trivial peculiarity of one parent, and prob- 

 ably of both, may be transmitted. This has led a number of zoolo- 

 gists to look upon the nucleus as a body containing specific elements 

 corresponding to those of the individual from which the nucleus has 

 come, but inheritance through the nucleus is no more a demonstration 

 of the existence of pre-formed elements of the male than are the gen- 

 eral facts of embryology a demonstration of pre-f ormation. All we can 

 legitimately conclude is that the substance of the nucleus is of such a 

 sort that it acts on the cytoplasm in a definite way, and determines, 

 in part at least, its differentiation. There has been steadily accumu- 

 lating evidence to show that during development there is an inter- 

 change of material between the nucleus and the protoplasm, and it is 

 not going far afield to conclude that the character of both nucleus 

 and protoplasm is altered by the interchange in material. If this is 

 admitted it is no more remarkable that a hybrid is midway between 

 its parents than that a parthenogenetic egg produces a form like that 

 of the individual from which it has come. 



