ORGANIC EVOLUTION 



19 



find it difficult to believe that such complex, yet 

 similar, things could have arisen independently. 



Because we can often arrange the series of struc- 

 tm-es in a line extending from the very simple to the 



Me.n 



Monkey Dog Sheep Horse 



Fig. 5. — Legs of five mammals, drawn to scale, to show homologous 

 parts. (After Leconte, from Romanea.) 



more complex, we are Si\)i to become unduly im- 

 pressed by this fact and conclude that if we found 

 the comjilete series we should find all the interme- 

 diate steps and that they have arisen in the order of 

 their comj^lexity. For example, there have appeared 

 in our cultures of the vinegar fly, Drosophila mel- 

 anogaster {fig. 6) over four hundred new types that 

 breed true. Each has arisen independently and sud- 

 denly. Every part of the body has been affected by 



