age and on conjugation leads to a unified result, and one that is in most respects in consonance with what we observe in higher animals. But in one respect there is a difference, and this brings us back to the question with which we began. Is death a necessary accompaniment of life? Do the life processes necessarily take such a course that they must lead to death?
To this question the work on the infusoria answers No! The evidence that was supposed to show that the life processes must gradually run down and end in death had been shown by the work of Woodruff not to lead to any such conclusion. Woodruff appears to be clearly justified in his recent statement that these organisms "have the potentiality to perpetuate themselves indefinitely by division," and my own studies on the effects of conjugation furnish the complement to this result, agreeing with it fundamentally.
All that Weismann meant by saying that such creatures are potentially immortal has shown itself correct. Death is not necessarily involved in life.
But why, then, in higher animals and in ourselves, even when there is no accident and conditions are good, do we find death coming as a natural end to life? Why should there be this tremendous difference in such an essential point between the lower organisms and the higher ones? Is there any possibility of mistake as to the necessity in the case of higher organisms?
To find a ground for this difference, we shall do well to follow the usual procedure in science, and examine other differences between these lower creatures and the higher ones, to see if these may not give us the clue. And here I touch upon a matter that had been fully developed by Minot and others; it is worth while to speak of it briefly, because work bearing upon the matter has recently appeared.
The most striking other difference between these lower organisms and the higher ones, is evidently the fact that in the higher organisms the body becomes large, complex and differentiated into a number of diverse parts; different cells of the body have taken on themselves different functions and different structures. This appears to, involve a correlative loss of the power of carrying on the fundamental vital processes; the cell that has become filled with lime, or that has transformed into muscle, no longer retains the vital elasticity of the cell in which the diverse functions remain well balanced. Products of metabolism are no longer perfectly removed; other processes necessary to life become clogged. The final result of this is a complete cessation of the processes; age and death follow upon differentiation. This, as you know, is the theory of Minot. According to it, the welfare of the individual cell is as it were sacrificed to that of the body as a whole, and this in turn involves the final destruction of the body itself, so that a
