90 Miscellaneous. 
capacity of forming a body-cavity, and yet its descendants have in 
some unaccountable mauner (entirely against the rules of Hiickel’s 
theory) managed to get one by some unexplained method. We do 
not see how it can be so confidently stated by Hackel that Echi- 
noderms have lost their original central nervous organ; there is no 
proof whatever of its once having existed. ‘There is as yet no proof 
whatever that the organs of sense (which, as had already been so 
often insisted upon by Agassiz, are not homologous in the different 
branches of the animal kingdom) have the same phylogenetic origin. 
When Hiickel says that the mouth of Echinoderms is not homolo- 
gous to the primitive mouth, we can only refer him to the memoirs 
of Miller, Metschnikoff, and myself on Echinoderm embryos for proof 
to the contrary. 
There seems no doubt, as Hickel insists, that to the majority of 
zoologists of the present day the idea of type is a very different 
one from that of type as understood by Baer and Cuvier. The 
probability of their original community of origin is hinted at from 
the many so-called intermediate forms, both living and fossil, which, 
though we may enroll them either in one great branch of the animal 
kingdom or another, yet show that we can no longer consider the 
great types of the animal kingdom as closed cycles, but must here- 
after regard them as holding to one another relations similar to those 
which the remaining categories of our systems have to one another. 
This change has principally been brought about by a better know- 
ledge of the embryology of a few well-known types. 
But what becomes of all the assumptions of Hiickel which form 
the basis of his Gastrea theory? They are totally unsupported ; 
and with their refutation must fall his theory; it can only take its 
place by the side of other physiophilosophical systems ; they are 
ingenious arrangements laboriously built up in the interests of special 
theories, which fall to the ground the moment we test them by our 
actual knowledge. That the time has not yet come for embryolo- 
gical classifications, the attempts of Hickel plainly show ; for they 
are in no wise in advance of the other embryological classifications 
which have preceded them: we get new names for somewhat differ- 
ent combinations ; but a truly scientific basis for a classification based 
upon the value of embryonic layers is at present impossible ; such 
attempts can be only speculations, to be proved or disproved on the 
morrow. 
What Hickel substitutes in the place of the accepted types of 
the animal kingdom is simply another view of these same types; 
and his Gastrea theory is in no danger of upsetting, at present at 
least, zoological classification as now understood. Indeed, if we 
need an ancestor for our phylum, why not at once go back to the 
cell? There we have a definite starting-point, a typical element 
which underlies the whole of the animal kingdom, and which forms 
the walls of Hickel’s Gastrula. Then we shall all be agreed; and 
when we frankly state that all organisms are derived from a pri- 
mitive cell and from its subsequent increase, we come within the 
range of positive knowledge, but we are unfortunately as far as 
ever from haying for that reason been able to trace a mechanical 
