THE MUTATION THEORY 313 
pretation of the behaviour of Ginothera Lamarcktana is 
not by any means an easy matter. 
As enunciated by de Vries, the theory of mutation 
amounts to a very complete and definite hypothesis. 
A large part of this author’s suggestions are, however, 
almost purely speculative, and for this reason we have 
treated the whole at somewhat less length than it per- 
haps deserves. Some of de Vries’ speculations are, 
indeed, more picturesque than convincing. 
Thus, de Vries regards the number of unit characters 
—each of which has arisen by a single mutation—to 
be quite limited, even in the highest organisms. 
Three or four thousand such characters, he thinks, 
may go to build up the hereditary endowment of 
the most complicated species. He further supposes 
a period of mutation to recur about once in 4,000 
years. Four thousand multiplied by 4,000 gives 
16,000,000—the number of years required to evolve 
the lords of creation from a ‘ primordial protoplasmic 
atomic globule.’ And he points out that this estimate 
is well within the limits of geological time as allowed 
by the physicist. In this way de Vries believes that 
his mutation theory removes a difficulty which besets 
the selection hypothesis—the difficulty, namely, of 
insufficient time. The selectionist may reasonably 
reply that the amount of change necessary to produce 
in 4,000 years, by the gradual method, a difference 
equal to that represented by a single unit character, 
might very well be quite imperceptible in a single 
generation. 
We may summarize our present conclusions as to 
