72 EXPLANATIONS. 



development of each line through its various 

 general grades of being is to be sought in an 

 internal impulse, the nature of which is unknown 

 to us, but which resembles the equally mysterious 

 impulse by which an individual embryo is passed 

 through its succession of grades until ushered into 

 mature existence. Geology shows us each line 

 taking a long series of ages to advance from its 

 humble invertebrate effluents to its highest mam- 

 malian forms ; and this I have ventured to call 

 " the universal gestation of Nature." 



The traces of this order of the animal kingdom 

 have been seen in all ages of science. Every 

 zoologist acknowledges the gradations and affini- 

 ties which appear amongst animals. Prompted 

 by what so palpably meets observation, many have 

 tried to range the various orders or families in one 

 line, or (to use the favourite phrase) chain of 

 being ; but they have always failed, which is not 

 to be wondered at. One cause why zoologists 

 have not up to this time thought of trying any 

 different arrangement, is the confusion arising 

 from the prevalence amongst many families of 

 parallelisms of structure, which have been regarded 

 as affinities, when in reality they are only identical 

 characters demanded by common conditions, or 



