OF PLANORBIS AT STEINHEIM. 29 



comparative uniformity of these stages wherever they have been observed, except in a 

 few extreme cases above described, and their almost exact parallelism in different individ- 

 uals of the same type, and by the observations of Barrande, Chalmas and the author among 

 the embryos of fossil Trilobites, Ammonites and Nautili. 



Why, then, are these embryonic characteristics usually so invariable,^ and why do they 

 resist even the efforts of the parasitic environment to crowd them out by the action of the 

 same law of heredity by which they first became embryonic ? 



If they first became embryonic by the action of the law of acceleration upon'the adult 

 characteristics of some ancestral form, all traces of which are now lost, why did they not 

 in their turn disajjpear ? 



An answer to this in detail must be deferred to a paper I am now preparing, an abstract 

 of which will, I hope, m due time appear in the Proceedings of the Boston Society of 

 Natural History. Here I shall try to show that this invariability rests not only upon the 

 power of the earlier and more embryonic organisms to resist change produced by the 

 environment, but also upon the greater protection from the working of physical causes 

 afforded to the young during the earlier periods of its existence. 



The environment is more uniform as regards temperature, food, and so on, in the egg, 

 than at any subsequent period. Starting with the common origin of the ovum, I think 

 it can be shown that the uniformity of the environment of the earlier stages, whether 

 held " in utero " or cast out to take care of itself, whether carried and protected in 

 pouches or at the breast, is not assumed without a sufficient basis of evidence. If it be 

 granted, that there is a growth force in organisms, which is the basis of all heredity, and 

 which resists the working of physical surroundings, tending to preserve the type 

 and sustain the uniformity of the organization, which limits variations and produces 

 parallelisms, the conclusion becomes inevitable, as in the Steinheim shells and the 

 persistent and embryonic types, that the nearer an animal is to the point of origm of its 

 type the stronger or more potent will be all of its uniformities, and the less subject to 

 variation from changes in the environment. 



If then we can join to this any general law of uniformity in the environment at early 

 stages, we have two efficient causes working in unison to maintain the stability in 

 geologic time, and the invariability in heredity of existing types, as shown by the 

 characteristics of the embryo. 



Deferring the proofs of this position to a future opportunity, it only remains to add by 

 way of caution, that I recognize in the term growth-force an expression of a purely 

 physical cause, which is shown by the fact that organic forms can and do habitually grow 

 in opposition to the forces of gravitation, as well as exist and perjDctuate themselves in 

 spite of the action of physical forces of the environment, which even under the most 

 favorable circumstances suffer them to exist but a very lunited time, killing them off in 

 inverse proportion to the innate powers of resistance, or growth force, manifested 

 by them. I do not pretend to assume that this force is antecedent, correlative, or 

 consequent upon the production of organic matter, I simply assume its active existence, 



1 Comparative invariability is here meant, for as I have more recent descendant forms. See also H. J. Clark, 



shown, Fossil Cephalopoda, Bull. Mus. Comp. Zooh, vol. "Mind in Nature," on the differences of the egg in different 



3, there is considerable variability in the earlier stages of types, etc., and authors on the structure of the egg at early 



Silurian Goniatites, as compared with the same stages in stages. 



