i 9 4 AGE, GROWTH, AND DEATH 



the top, the cells are cast off. I have already spoken 

 of the enormous loss of cells from the lining of the 

 digestive canal which occurs throughout life. 



I could multiply these instances almost indefinitely, 

 but perhaps it will be better to call your attention to 

 an illustration of quite a different sort. We know 

 that in order to have a very complex organisation, 

 the number of cells in the body must be very large 

 indeed. Obviously a small insect, a mosquito or a 

 little beetle, whatever it may be, is not big enough to 

 have a great many cells ; and, unless it has a great 

 many, it cannot attain the differentiation of compli- 

 cated organs such as we possess. Now, the lower 

 animals are born, so to speak, early, and as soon as 

 they hatch out, they have to support themselves. We 

 see that, for instance, in caterpillars. They are born 

 very little creatures, but each tiny caterpillar must 

 take care of itself, obtain its own food, move about to 

 that food, must, when the food is swallowed, digest it, 

 and must carry on the correlated functions of secre- 

 tion and excretion ; it must breathe. In order to do 

 all this the larva, or young caterpillar, to follow our 

 special instance, must have some differentiation al- 

 ready established ; but, as we have learned, differen- 

 tiation impedes growth. In other words, in such a 

 larva the multiplication of cells is held back by the 

 very demands of the condition of its existence. If it 

 is to have organs which are to function, it must have 

 differentiated parts, and, if it is differentiated, its 

 growth power must be sacrificed. 



