No.1.] VARIATIONS IN LIMULUS POLYPHEMUS. c9 
organs. But there is nothing to indicate with certainty 
whether the initial cause of variation is due (1) to a variation 
in the combination, or the quantity, or the quality of the 
original constructive materials, or (2) to the variation in condi- 
tions external to the ovum, or (3) to a combination of both. 
But as very divergent forms appear among eggs kept under 
apparently the same conditions, it would seem more than likely 
that the first set of causes are the real ones. 
Variations in the unfertilized ova, and in the spermatozoa 
or polar globules, at once arise before the mind in their 
familiar attitude, as factors in some way connected with the 
phenomena of variation. And there is the whole infinitely 
complex Weismannian mechanism, with its endless army of 
corpuscular brownies, on whose sins of omission or commission 
we may easily throw the responsibility. 
But these corpuscular theories fail to explain anything. Their 
agents come or go at the beck and call of him who commands 
them. We are in the end left with the sterile formula, that 
this or that organ is as it is because the necessary corpuscles 
were there to make it so ! 
All the variations so far observed can be traced back to 
variation in the relative rate of growth, specialization, and 
degeneration in the numerous groups of cells that constitute 
the embryo. The problem then is to explain why one group 
of cells grows faster or slower in one embryo than in another, 
or why it grows at all. But the conditions determining 
differential growth are very complex, and may be different in 
each particular case. No simple general statement will suffice. 
We have passed that stage where it is enough to know that a 
machine is made of iron and run by electricity. We must 
know what electricity is, and the structure and bearing of 
every pin, screw, and wheel in the whole mechanism. It is 
needless to say that we are very far from having any such 
knowledge even of the simplest bit of living matter. 
The facts here presented enable us to catch a glimpse of 
embryological processes under new conditions and from a 
different standpoint, and while many of them increase rather 
than diminish the existing difficulties, they will perhaps, in 
