212 



HABIT AND INTELLIGENCE. 



[chap. 



Species are 

 permanent 

 varieties. 



Varieties 

 are most 

 numerous 

 where 

 species 

 are so. 



Aberrant 

 genera are 

 poor in 

 species. 



If the development theory is true, and if all species 

 whatever, or all the species of any wide group, are de- 

 scended from the same original ancestor, then, as Darwin 

 has expressed it, " species are only well-marked and per- 

 manent varieties ; " and the species of a genus stand to 

 each other in the same relation as do the varieties of a 

 species. Such is shown to be the case by a great amount 

 of evidence which Darwin has collected, and arranged in a 

 way that could only have been done by one combining his 

 vast knowledge of the details of natural history with his 

 power of seizing on the essential relations of a subject. As 

 his " Origin of Species " is a well-known and very readable 

 book, it is only necessary to state the following facts in the 

 barest outline. 



" Where many large trees grow we expect to find sap- 

 lings ; " and when a form has been varying much within 

 recent geological time, so as to form many species belong- 

 ing to one genus, we may expect to find that it is still 

 varying. Such is the fact : if large genera (that is to say, 

 genera containing many species) are compared with small 

 ones, it will be found that, on the average, a greater number 

 of the species of the large genera than of the small ones 

 present varieties ; and those which do present varieties, 

 present a greater number of them. These results, of course, 

 are not uniform, only average ones.-^ What is a fact of 

 exactly the same order, aberrant genera (that is to say, 

 genera that are very peculiar, and not nearly akin to any 

 others), are nearly always small genera.^ On the theory of 

 development, an indefinite number of intermediate forms 

 must have existed formerly. A genus has become aberrant 

 by the extinction of all the species and genera that formerly 

 connected it with other known forms; and as the same 

 causes, whatever they are, that determine the increase or 

 the extinction of a species or of a genus usually act nearly 

 alike on kindred species and genera, the causes that make 

 a genus aberrant by destroying the kindred genera will 

 also make it poor in species by destroying many of its 

 species. Such genera are Ornithorhynchus and Lejndosiren. 

 ^ Darwia's Origin of Species, p. 65. ^ ibj^i p 508, 



