38 ZOOLOGICAL PHILOSOPHY 



Not only many genera but entire orders, and sometimes even classes, 

 furnish instances of almost complete portions of the series which I 

 have just indicated. 



When in these cases the species have been arranged in series, and 

 are all properly placed according to their natural afiSnities, if you 

 choose one, and then, jumping over several others, take another a little 

 way off, these two species when compared will exhibit great differ- 

 ences. It is thus in the first instance that we began to see such of 

 nature's productions as lay nearest to us. Generic and specific dis- 

 tinctions were then quite easy to establish ; but now that our collec- 

 tions are very rich, if you follow the above-mentioned series from the 

 first species chosen to the second, which is very different from it, you 

 reach it by slow gradations without having observed any noticeable 

 distinctions. 



I ask, where is the experienced zoologist or botanist who is not 

 convinced of the truth of what I state ? 



How great the difficulty now is of studying and satisfactorily 

 deciding on species among that multitude of every kind of polyps, 

 radiarians, worms, and especially insects, such as butterflies, Phalaena, 

 Noctua, Tinea, flies. Ichneumon, Curculio, Cerambix, chafers, rose- 

 chafers, etc. ! These genera alone possess so many species which 

 merge indefinably into one another. 



What a swarm of mollusc shells are furnished by every country 

 and every sea, eluding our means of distinction and draining our 

 resources. 



Consider again, fishes, reptiles, birds and even mammals ; you 

 will see that except for gaps still to be filled, neighbouring species 

 and even genera are separated by the finest differences, so that we have 

 scarcely any foothold for setting up sound distinctions. 



Is there not an exactly similar state of affairs in the case of botany, 

 which deals with the other series, consisting of plants ? 



How great indeed are the difficulties of the study and determina- 

 tion of species in the genera Lichen, Fucus, Carex, Poa, Piper, 

 Euphorbia, Erica, Hieracium, Solanum, Geranium, Mimosa, etc., etc. 



When these genera were constituted only a small number of species 

 belonging to them were known, and it was then easy to distinguish 

 them ; but now that nearly all the gaps are filled, our specific 

 differences are necessarily minute and usually inadequate. 



Let us see what are the causes which have given rise to this un- 

 doubted state of affairs ; let us see if nature affords any explanation, 

 and whether observation can help us. 



We learn from a number of facts that, according as the individuals 

 of one of our species change their abode, climate, habits, or manner 



