OTHER THEORIES OF SPECIES-FORMING. 275 



and finally of explaining the definite cases of ultra-develop- 

 ment of parts and species beyond the point of advantage even 

 to such unfavourable degrees as lead to death and extinction. 

 Palaeontology 8 reveals to us the one-time existence of ani- 

 mals, of groups of animals, and of lines of descent, which 

 have had characteristics which led to extinction. The un- 

 wieldiness of the giant Cretaceous reptiles, the fixed habit 

 of life of the crinoids, the coiling of the ammonites and 

 nautili, the gigantic antlers of the Irish stag all these are 

 examples of development along disadvantageous lines, or to 

 disadvantageous degrees. The statistical studies of varia- 



o o 



tion have made known numerous cases 9 where the slight, as 

 yet non-significant (in a life-and-death struggle) variation 

 in pattern of insects, in dimension of parts, in relative pro- 

 portions of superficial non-active structures, are not for- 

 tuitous, that is, do not occur scattered evenly about a mean 

 or mode according to the law of error, but show an obvious 

 and consistent tendency to occur along certain lines, to 

 accumulate in certain directions. Many biologists see in 

 variation and in species-forming certain determinate char- 

 acteristics exhibited by, or lines or paths being followed by 

 all or most of the individuals of successive generations ; and 

 see in descent certain phenomena of forthright progressive 

 movement which they find selection based on utility unable 

 to explain. Various theories to account for this apparent 

 orthogenetic, but not ortho-selective, development have 

 therefore been proposed by biologists, most of which 

 theories and most of which biologists are to be looked on as 

 antagonistic to the selection theory. For if a theory of ortho- 

 genesis is sufficient to explain those lines of variation and 

 development not explicable by selection, it usually seems to 

 its maker to be sufficient to explain other lines of evolution. 

 It may very likely occur to some that in speaking of ortho- 

 genetic development as contrasted with descent governed by 

 selection we are making a distinction without a difference, 



