OF ORGANIC NATURE. 25 



have arrived at the conviction that there are, — some 

 think five, and some seven, — but certainly not more 

 than the latter number — and perhaps it is simpler to 

 assume five — distinct plans or constructions in the 

 whole of the animal world ; and that the hundreds of 

 thousands of species of creatures on the surface of the 

 earth, are all reducible to those five, or, at most, seven, 

 plans of organization. 



But can we go no further than that ? When one 

 has got so far, one is tempted to go on a step and in- 

 quire whether we cannot go back yet further and bring 

 down the whole to modifications of one primordial unit. 

 The anatomist cannot do this ; but if he call to his aid 

 the study of development, he can do it. For we shall 

 find that, distinct as those plans are, whether it be a 

 porpoise or man, or lobster, or any of those other kinds 

 I have mentioned, every one begins its existence with 

 one and the same primitive form, — that of the egg, 

 consisting, as we have seen, of a nitrogenous substance, 

 having a small particle or nucleus in the centre of it. 

 Furthermore, the earlier changes of each are sub- 

 stantially the same. And it is in this that lies that 

 true " unity of organization " of the animal kingdom 

 which has been guessed at and fancied for many 

 years ; but which it has been left to the present time 

 to be demonstrated by the careful study of develop- 

 ment. But is it possible to go another step further 

 still, and to show that in the same way the whole of 

 the organic world is reducible to one primitive con- 

 dition of form ? Is there among the plants the same 

 primitive form of organization, and is that identical 

 with that of the animal kingdom ? The reply to that 



