Lecture XXX. 271 



It now remains to summarise very briefly the salient points of 

 the botanical evidence which has helped to lead scientific opinion 

 of to-day to accept the theory of the Origin of Species by Descent, 

 or as it is often called the theory of Evolution. 



We will consider the evidence under three heads : (i) Evidence 

 from Morphology. (2) Evidence from Distribution. (3) Evidence 

 from the Geological Record. 



First with regard to morphological evidence. 



(a) Argument from Classification. Even when the dogma of 

 the immutability of species was generally held, naturalists in ex- 

 amining and recording the characters of species recognised that 

 groups of these species had features in common, and were dis- 

 tinguished by these features. The smaller groups of species were 

 called genera. Thus the genus Ranunculus is a group of several 

 different kinds of buttercup, e.g. the species Jt. bulbosus, R. repens, 

 etc. It was noticed that the species naturally grouped them- 

 selves so that those of one genus resembled one another more 

 closely than the species of different genera. Similarly these 

 genera could be arranged in larger groups, often called families. 

 These two terms are in themselves significant. They indicate 

 that those who applied them had in their minds the idea of 

 racial and family relationship. The families were associated 

 into orders and the orders into still larger assemblages called 

 classes. In arranging the various kinds of plants and animals in 

 the classification of species, genera, families, orders, classes, etc., the 

 relationship of the various species in a genus, or of the genera in 

 a family was purely ideal, and the grouping of similar forms under 

 certain names was purely formal and was merely a matter of con- 

 venience in the minds of those who believed that species are immut- 

 able. Evidently if the different species had been formed as special 

 acts of creation and were capable of no subsequent transformation into 

 other species, no genetic connection linking individual species was 

 possible. Thus classification was regarded as arbitrary, depending 

 solely on the distinguishing criteria selected. The unsatisfactori- 

 ness of this view, coupled with the observation that family re- 

 semblances in man are based on genetic relationships and the 

 great probability (as was strongly held by many who asserted 

 the immutability of species) that racial characters indicate common 

 ancestry, led biologists to reject the theory of special creation. 

 They saw in the possibility of the classification of organisms a 

 strong reason for assuming common descent for those species which 

 closely resembled one another, and for believing that the closeness 

 of resemblance is in general a measure of the closeness of the forms 



