THE EVOLUTION OF ORGANIC TYPES IN TIME. 99 



would bring to light allied types from which we might suppose it to 

 have descended. 



4. Relative Persistence of Species in Time. — The duration in time, 

 or " vertical range " of species, varies greatly in different cases. Some 

 species have an extraordinarily extended range, sometimes passing 

 through two or three geological systems, and in such cases they 

 generally exhibit numerous varieties. This is the case, for example, 

 with some of the Brachiopods, such as Strophomena rhomboidalis 

 and Atrypa reticularis. Others, again, are singularly restricted in 

 their range, and do not pass beyond the limits of a single subdivision 

 of a system, or, it may be, even of a single band or zone. No case 

 is known in which a species which has once fairly died out has re- 

 appeared at a later period, but there is no absolute impossibility in 

 the separate evolution of the same specific type at two separate 

 periods. As a general rule, it is the animals which have the lowest 

 and simplest organisation that have the longest range in time, and 

 the additional possession of microscopic or minute dimensions seems 

 also to favour longevity. Some of the Foraminifera, for example 

 {e.g., Saccanwiina Carteri), seem to have survived, with little or no 

 perceptible alteration, from the Ordovician period to the present day. 

 On the other hand, large and highly organised animals, though long- 

 lived as individuals, rarely seem to live long as species, and have, 

 therefore, usually a restricted vertical range. Some genera, as some 

 species, are short-lived ; whereas others extend through a succession 

 of geological periods with extraordinarily little modification. Among 

 these " persistent types " may be specially mentioned the genus Lin- 

 gula among the Brachiopods, and Nautilus (in the wide sense of the 

 name) among the Cephalopods, of which the former commenced in 

 the Cambrian and the latter in the Ordovician, and both of which 

 are represented by living species. 



5. Relative Range of Morphological Types in Space. — The range of 

 particular morphological types in space is as variable as it is in time. 

 Some forms appear to be wholly restricted to some particular area, 

 or, possibly, to a single locality ; while others have an enormous 

 range, and are found at very widely distant points of the earth's sur- 

 face. In a general way, the types which have a wide range in time 

 have also a wide range in space. Thus, species of Brachiopods 

 like Strophomena rhomboidalis and Atrypa reticularis, which range 

 through more than one geological system, have likewise a very 

 extended geographical distribution. Still, a species which is con- 

 fined to a single system may have an enormous range in space — as, 

 for example, the common Producta semireticulata of the Lower 

 Carboniferous ; while in some cases types which are restricted to a 

 single " zone," like certain Graptolites and Ammonites, are found to 

 range over very wide areas. The apparently simultaneous appear- 



