the Embryos and Seedlings of the Cactaceae, 471 
and single growing-points to the tubercles suggest an origin 
from Echinocactns in some division other than Thelocactus , 
but at present I have no further data. It must come from 
near a form with the ribs replaced by tubercles. 
Since my materials were chosen especially to represent the 
embryology of the genera and leading groups, and hence only 
rarely include closely related species, it does not give a fair 
idea of the extent to which embryology can be used in this 
family as a clue to the relationships of closely related or 
doubtful species. Obviously a large amount of accurate data 
and a thorough knowledge of the principles controlling form- 
changes must be accumulated, and the present study needs 
to be supplemented by a minute study of the embryology 
of groups of related species, and this I hope to make in the 
form of morphological life-histories of certain species. I have 
no doubt that the very stable characters of the embryos, and 
the remarkable way in which the epicotyls so often show the 
characters of the immediate ancestry, will make them of great 
systematic importance. And when to this we add the fact 
that most of the genera and many of the species in this new 
family are still in the nascent state, and that most of the 
connecting links are still in existence, I think it will be 
possible to recover the phylogeny of genera and species in 
this family to an altogether rare degree of completeness. 
