1876.] Haeckel’s Gastrea Theory. “3 
HAECKEL’S GASTRAZA THEORY. 
BY ALEXANDER AGASSIZ. 
a ai HAECKEL has just published in the Jenaische 
Zeitschrift} a second paper on the gastræa theory, devoted to 
answering the many attacks to which it has been subjected. It 
is fortunately free from the personalities which disfigure so many 
of Haeckel’s productions, and consists mainly of new theories and 
new interpretations of well-known embryological facts. Haeckel 
now endeavors by a most ingenious theory to explain the phe- 
nomena of segmentation, which (according to him) conceal the 
original unity of the gastræa in the different classes of the animal 
kingdom. As Haeckel now presents the gastræa theory it would 
be difficult to recognize it as he and his followers formerly under- 
stood it. 
It is unfortunate that Haeckel should feel obliged to coin so 
many new terms, for unless the reader can throw himself, heart 
and soul, into Haeckel’s position, he will-hardly feel inclined 
to master the delicate shades of meaning which a difference in 
prefix or termination involves. They undoubtedly contribute to 
the terseness of the text, but are so numerous that the reader can 
scarcely be expected to carry in his mind the necessary vocabu- 
lary, much of it dating back to the Generelle Morphologie. 
Haeckel has made an important admission in going back for 
his starting-point to the egg (as the opponents of his theory 
urged), and attempting to trace how far segmentation can be 
influenced by natural selection; he has of course seen the diffi- 
culty, of which all embryologists are aware, of accounting through 
such a cause for the vital divergence observed in the segmentation 
of closely allied groups, leading eventually to the same result. It 
_ 18 difficult even in the wildest flight of imagination to frame a 
theory to account not only for these radical differences of devel- 
opment in the ancestral eggs, living in the same medium, sub- 
ject to identical influences, but also for their transmission by in- 
heritance. Haeckel’s explanation of the causes which have led 
to the concealment of the descent of the gastrula is that only 
those embryonic processes which can be traced directly to a 
former independent ancestral form, and can be inherited, are of 
primary importance for the recognition of genetic connection, 
while those embryonic phenomena which are due to adaptation 
of the embryo or larval condition can claim only a very secondary 
1 Die Gastrula und die Eifurchung der Thiere. 
