OF PLANORBIS AT STETNHEIM. 19 



The parallelisms of the species or forms in the different series are, however, produced 

 by characteristics, which, as we have noted above, are not inherited from PI. levis, and could 

 not have been inherited from, any previously existent species, since they originate inde- 

 pendently in the forms of each series. This condition of affairs casts doubt upon these 

 unqualified statements of the paramount influence of the forces of growth as stated above, 

 and leads to the following question : Are these parallelisms adaptations, and can they 

 possibly be attributed to the direct action of the uniform external environment upon the 

 forms of the different series ? This can hardly be answered in the Steinheim locality, but 

 still there are indications that here, as in other groups, these parallel characteristics are 

 not due to similar inorganic influences. The lower forms probably arose in the First 

 Period if my observations are correct, and only a part of them, the PI. trochiformis in 

 the Fourth Series, the forms of the First Series and its three sub-series during the Second 

 Period, in the pits, and those of PI. crescens in the Second Series were evolved, 

 during the same time. It does not appear, then, that the inorganic influences, 

 which were probably very dissimilar, if we can judge by the deposits, during these 

 two periods, could have been the cause of the representation. Fortunately, how- 

 ever, the phenomenon of representation is quite common in the animal kingdom, 

 and we can look elsewhere for a solution of this question. I have described a 

 large number of species of Ammonites, and in no case was it possible to attribute the 

 independent production of similar forms in distinct series to the action of similar physical 

 environment. On the contrary, the most remarkable cases of parallelism took place 

 frequently in series occurring in different formations and distinct faunae, just as the 

 remarkable parallelism of the Marsiipials with the rest of Mammalia. The unquestionable 

 case of the Marsupials of Australia, may serve as a means of estimating the effects of the 

 environment. While this certainly cannot be said to determine either the growth of the 

 individuals in parallel lines, or to be the cause of the production of the parallel forms, it 

 may nevertheless be essential to the full exhibition of both phenomena. 



It must be remembered that in the Marsupials we have, probably, the lowest 

 mammalian type, as well as the oldest, and the greater number of representative forms 

 which we now find in Australia, are characteristic of the present period, and they 

 are not found in the fossil European, nor in the existing or fossil American forms. There 

 is, then, something peculiar in the environment in Australia, which makes it possible 

 at least for these forms, which represent Rodentia, Carnivora, etc., to be produced 

 there. It is evident that, if the land had already been possessed by these classes of typical 

 mammals, or if they had not belonged to the base of the mammalian stock, no such 

 expansions of the marsupial type would have been possible. It required these two 

 elements ; the growth force of a basic mammalian type and room for it to grow 

 and reproduce, or a free environment. This was the case also in the Steinheim lake. 

 Planorhis levis is an immature or low form, the field into which it entered was free, and 

 it developed all its latent growth force, in order to fill it with species. In the same way 

 the Ammonites did not exhibit their greatest expansion until after the lowest competing 

 type. Nautilus, having expended its growth force in the Carboniferous, began to die out. 



This hypothesis then would attempt to account for the production of so many similar 

 forms in distinct series, simply by the fact, that the series had room to expand, or to grow 



