' THE BIOLOGICAL PROBLEM OF TO-DA Y 



orphism and polymorphism. These seem to me 

 V to show how very different final results may grow 

 from identical rudiments, if these, in early stages 

 /* of development, be subjected to different external 

 / influences. 



Finally, I have a little to say about the sexual 

 dimorphism that occurs so generally in the animal 

 kingdom. 



Nearly all kinds of animals appear as male or 

 'as females. These differ from each other not only 

 in that they produce eggs or spermatozoa, but 

 frequently in a number of more or less striking 

 characters affecting different parts of the body, and 

 known as secondary sexual characters. In fact, 

 the difference between the sexes may be so great 

 that a systematic naturalist, unacquainted with the 

 mode of development of the creatures, might place 

 them in different species, genera, or even families, 

 on account of the striking differences in external 

 characters. 



As an instance, take Bonellia, a gephyrean, the 

 strange case of which has been remarked upon by 

 Hensen and by Weismann. The male is about a 

 hundred times smaller than the female, in the 

 respiratory chamber of which it lives as a kind of 

 parasite, and appears, so far as outward shape goes, 

 more like a turbellarian than a gephyrean. None 

 the less, male and female are alike not only while 

 they are in the egg, but as larvse, and it is only 

 towards the period of sexual maturity that the 

 great difference between them begins to appear. 

 So also is it with the dwarf males of the cirripedes. 



