20 HYATT ON THE TERTIARY SPECIES 



and reproduce to the fullest extent in this field ; that they did so in precise accordance 

 with the laws of growth, and the succession of characteristics in the individual. This is 

 very evident from the fact that Planorbis exhibits no such tendencies in other localities 

 where it is surrounded by competing forms. It has become plain probably to the " begeis- 

 terte Darwinianer " by this time, that this is in fact an application of the law of natui'al 

 selection, but he will also see that it is accompanied by such impoi-tant modifications, that 

 it is reconciled with the laws of growth. Thus it may be said that the struggle for exist- 

 ence, and the survival of the fittest, is a secondary law grafted ujjon laws of growth, and 

 governed by them in all its manifestations. 



The law of natural selection, as generally understood, assumes in the first 

 place the existence of an animal type, of its descendants, and of a tendency to vari- 

 ation (indefinite and unlimited) in every one and all of these descendants, from which (an 

 indefinite and unlimited) selection may take place during the struggle for existence 

 between competing forms, destroying the weak and pei'mitting only the strongest and 

 fittest of these variations to survive. 



The truth is. as fiir as my studies have gone, that there is no such thing as indefinite or 

 unlimited variations in any species. They may perhaps be considered innumerable, 

 but they are not indefinite or unlimited. This obvious proposition, if admitted, leads at 

 once to the question, what are the limits within which a species may vary ? Making 

 s2Decial studies for this purpose among the Ammonites, the limits of variation in the 

 species have been found to correspond to the growth changes in an individual. Some 

 individuals may retain a portion or a large part of their earlier developed characteristics 

 (not embryological), some may make considerable modifications in their hereditary adult 

 characteristics, amounting even to new additions in many instances ; some may occupy the 

 other extreme, and either as diseased individuals, or as individuals under circumstances 

 very vuifavorable to normal growth, show premature senile and retrograde metamoi'phoses 

 and distortions. This also is a picture of the grander variations of any large or 

 small group of Ammonites, and of the present group of Planorbis. PI. levis may vary 

 from the equiumbilicated discoidal form, to the unequiumbilicated form similar to PI. 

 oxi/.stomus, or the more depressed whorls of PL 'X'.'is'% and each of the varieties may 

 have minor sub-varieties founded upon innumerable minor differences in the spiral, 

 more or less angular outer sides, and so on, but there is evidently a well defined 

 law in their develojDment. The variations consist in the retention of the earlier 

 or young form with no additional progressive characteristics, or if these are added 

 they consist of modifications or exaggei*ations of some part, found more or less 

 developed in other forms, whether these occur in the lakes of America or other continents. 



It has already been shown that the representative forms were divisible into two kinds. 

 Those whose similarities could be accounted for, because they differ very slightly from 

 PL levis, retaining in part its form and smooth whorls, and those subsequently prodviced 

 which were new in the Steinheim lake, and, that the former, which are due to the reten- 

 tion of ancestral characteristics, are replaced by the latter. 



Thus the equiumbilicated discoidal form is lost entirely in PL tenuis, PL iriquetrus, 

 and PL crescens, except in the young of some specimens. In PL cUscoideus, it is not 

 even found in the young which are as_ynnmetrical at all ages, except perhaps the young- 



