THE CONTINUITY OF LIFE 19 



logical investigation has unearthed, many forms have been 

 found which are the actual ancestors of groups now distinct, 

 and they have thus been of the greatest value in tracing out 

 phylo genetic relationships. Others, however, represent a 

 series of forms which developed, culminated and became ex- 

 tinct before modern times, thus presenting a group of great 

 value to the student, but having no bearing upon the present 

 discussion. 



Perhaps the most famous of the ancestral forms found in 

 a fossil state is the Archccopteryx, a definite transition be- 

 tween reptiles and birds. Of this, two specimens were discov- 

 ered in the lithographic slate quarry at Solenhofen, Germany. 

 Others, of almost equal importance, have assisted greatly in 

 suggesting the relationship between amphibians and reptiles, 

 and have furnished clews to the proper arrangement of the 

 orders of living mammals. As illustrations of large groups 

 of animals whose history lies wholly in the past may be men- 

 tioned the trilobites, a group of crustacean-like articulates, 

 which became wholly extinct at the end of the Palaeozoic 

 Age, and the ammonites, a group of cephalopod molluscs. 



V. The relative amount of structural difference between 

 any two divergent forms is proportionate to the amount of 

 contrast between their environments, and not necessarily to 

 the amount of time that has elapsed since their divergence 

 from the common ancestor. 



That time has in itself no power to modify an animal 

 species is shown by the slight differences that exist in some 

 cases between certain living forms and their fossil allies. 

 Perhaps the most conspicuous example of this is the brachio- 

 pod, Lingula, a worm enclosed in a bivalve shell. This form 

 has existed from the earliest Silurian times to the present day, 

 and yet there are hardly sufficient differences between the 

 earliest fossil Lingula and those now alive to allow them to 

 be treated as distinct species. As a rule, however, successive 

 geological periods show almost a complete change in their 

 fauna and flora, and most of the modern forms are quite 

 recent in origin. 



The persistence of ancestral types in a slightly modified 



