APPENDIX C 263
their
somatic origin. And if they be not somatic, with the actual facts of
development as they are, with the pre-embryonic appearance of the germ-cells,
a direct development is inconceivable, is as impossible as epigenesis.
On
the one hand, there are the three doctrines of somatic origin of germ-cells,
direct development, and epigenesis; on the other, a like number, morphological
continuity of germ-cells, antithetic alternation of generations, and evolution,
or unfolding.* All the known facts of development support the latter trilogy.
It is out of question to discuss, on the present occasion, all the bearings of
these three doctrines, or even to foresee these. Of necessity they are
connected together; each is an essential part of the theory of development.
Outside embryology and zoology their bearings upon scientific problems will
undoubtedly be great, but here it may be instructive to point to one or two
little things, which they help to explain in zoology and embryology. The
position of the sponges, and their mode of development, have been never-ending
themes of discussion with zoologists, while the evolution of the mammalian
trophoblast has not been less fruitful of speculation. For brevity, the facts
may be placed in tabular form:
LIFE-CYCLES
Name. |
Asexual generation |
Sexual generation |
Nemertine. |
Pilidium |
Nemertean Worm |
Sponge |
Sponge-larva and adult. |
Absent |
Sea-urchin |
Pluteus |
Sea-urchin |
Hydra |
Hydra |
Absent |
Hydroid polype (Campanularia). |
Hydroid colony |
Medusa |
Raja batis |
Blastoderm, transient
nervous system, etc |
Skate |
Mammal (e.g., man or
rabbit.) |
Trophoblast (chorion). |
Mammal |
* Differs from the older views in this way: The evolution of Bonnet,
Haller, and others, was an evolution with preformation, that of Weismann is an
evolution with predestination.