OF PLANORBIS AT STEINHEIM. 21 



est zones of growth in the shell. This decrease is evidently brought about by the prepo- 

 tency of the newly introduced tendency to ing-ease the spirality, and develop the 

 square form of the whorl and the sulcations and carinations. 



The retrogressive series, as may be readily seen, have an increase in the adult 

 retrogressive characteristics, which obeys the same law ; the farther removed the species 

 is from the original form the less it is apparent, either in the young or in any of the adult 

 forms. Thus, in following up the series we find, that in any one form during the adult 

 stages the representative characteristics displace the ancestral characteristics in inverse 

 proportion to the affinity of the forms in which they appear for the ancestral form. 



This law is applicable even to those resemblances occurring between the forms of the 

 old and young, such as have been traced between the oldest and youngest stages of the 

 individual among the Ammonites, by D'Orbigny and the author, and by many authors 

 between Baculites and Orthoceras. These resemblances are accompanied in these cases, 

 as in man, where there is considerable resemblance in the form of the body and the 

 parts at the extremes of life, by entirely distinct structures, and are evidently due to the 

 partial or entire absence of parts and organs. In the young this is found only before 

 or during the stage of development in the parts ; in the old however, only after or 

 during the stages of absorption of the parts. The retention of the cylindrical or semi- 

 cylindrical whorl in the adult of denudatus and dlstortus, are precisely comparable 

 with such geratologous characteristics. The extreme young are closely coiled and 

 similar to the young of PI. levis, and the subsequent aspect of the shell is bi-ought 

 about by retrogressive changes counteracting the normal tendency of the growth. They 

 are not arrests of development, but geratologous metamorphoses. True arrests of devel- 

 opment and reversions, if the latter can really be sejDarated from the former, are precisely 

 the exceptions which are needed to show the iniiformity of the law under ordinary 

 conditions, and its subordination to unfavorable or extraordinary external conditions. 



This law is equally applicable to the parallelisms of individuals of the same brood, same 

 variety, species, genus, or family. The differences which appear in the individual 

 adults, and which distinguish them at this stage from their own young, or their own 

 embryos, are for the most part those which show their affinity to other individuals of the 

 same brood, variety, species, genus, and family. 



These new or differential characteristics replace those of the earlier stages, which, as is 

 well known, are inherited from ancestors, who, with the exception of animals having the 

 larval stages much prolonged, have first acquired them during growth in their adult 

 stages. In fact, one cannot understand such series as are here shown, or as 

 may be constructed from a study of the affinities of animals, when arranged with 

 a due regard to the embryology, geological surroundings in different formations and 

 occurrence in time, and their adult characteristics, without assuming continuity of 

 descent. This being granted, all observations show that one and the same general law 

 covers all series, whether retrogressive or progressive, namely, that the representative 

 characteristics of the individuals and their differences in structure at the adult stage are 

 inversely proportional to their relative removal in time, and the surrounding conditions 

 or environment, from the egg or from some assmned or observed parent type. 



