280 DARWINISM TO-DAY. 



amples to show that a definite or determinate direction of 

 modification may be noted in all the sub-groups of a large 

 family, although appearing in unequal degree in different 

 species. Examples are the reduction of the hind toes among 

 the Artiodactyls which has continued in several genera 

 (giraffe, camel, llama) up to a complete disappearance, and 

 the modification of the originally single genital duct into a 

 double and finally triple one, as occurs in both the Pul- 

 monata and the Opisthobranchiata. Often a progressive 

 development can, on mechanical or physical grounds, come 

 about by a modification in but one direction, and may lead 

 thus to convergent changes, as the development of a lens in 

 a pigment fleck in the case of many unrelated lower animals, 

 the similarity of the heart in crocodiles, birds, and mammals, 

 the appearance of a placenta with Pcrmales among the 

 Marsupials and also among the Placentalia." 



2. The numerous "excessive structures" which are de- 

 veloped far beyond the limits ofusefulness. Examples, the 



tusks of the wild boar (Babirnssa alfurus) ; the 

 Over-speciali- giant horns of many wild sheep and goats; 



the enormously elongated thread-like neck of 

 several Rhynchophorous beetles, as Apoderus tenuissinius, 

 etc. ; the absurdly long eye-stalks of such crustaceans as 

 Macrophthalmus laterillei and Podophthalmus vigil; Meso- 

 plodon, whose mouth can be opened but a little when 

 the animal is full grown because on each side an under 

 tooth grows around the upper jaw. Such "excessive 

 structures" have undoubtedly led to the dying out of many 

 former species : examples, the tusks of the mammoth, the 

 antlers of the Irish stag, the canines of Smilodon neog&us. 



3. "The constitution, or actual chemical composition of the 

 body permits, in many cases, changes only in few directions. 

 The animal or plant breeder may by no means produce any 

 wished-for form or colour. No one has yet succeeded in 

 producing a blue Maiblume, a grass with divided leaves, a 



