-76 BIOLOGICAL LECTURES. 



separate bones for weeks, measuring, and writing descriptions 

 of them, I am always more or less surprised by the appearance 

 of the skeleton, when the bones are laid together in their natural 

 position ; there is always some feature of proportion which had 

 eluded attention. 



There is another reason which makes it highly desirable to 

 have a large number of individuals representing each species, 

 .and that is the very deceptive effects of even slight crushing 

 and distortion of the bones. So great is the pressure of 

 the overlying weight of sediment, even in undisturbed and 

 horizontal strata, that the bones are frequently somewhat dis- 

 torted or crushed. No one who has not examined a suite of 

 specimens can understand how totally the appearance of a fossil 

 may be changed by crushing, and the change may be so 

 wrought as to seem normal, except after a very careful exami- 

 nation. Two skulls of the same species, one of which has 

 been compressed laterally and the other vertically, will look so 

 different that at first it seems absurd to refer them to the same 

 animal, and several species, to put it mildly, have been estab- 

 lished on characteristics due to this process. To correct the 

 false impressions due to distortion, it is desirable to have many 

 specimens, and, even if none of them is quite symmetrical, a 

 careful comparison of the effects of crushing in different planes 

 will enable the observer to eliminate those effects and to recon- 

 struct the normal form of the species. 



A suite of well-preserved specimens from successive geo- 

 logical formations gives the material from which phylogenetic 

 series are to be reconstructed, and if the material is abundant, 

 and the series not interrupted by gaps, the results of careful 

 and conscientious work may be accepted with confidence. 

 Phylogenies, as hitherto made, have usually been confined to 

 genera, which give results too vague for many important pur- 

 poses; but already an encouraging beginning has been made 

 in constructing phylogenies of species. In the modern way of 

 collecting the exact level of every specimen in the strata is 

 carefully recorded, and thus it becomes possible to trace the 

 successive modifications, even of a species, through a few hun- 

 dred feet of beds which were uninterruptedly deposited. This 



