312 ANNUAL OF SCIENTIFIC DISCOVERT. 



remote ancestor of a given form? Each of us, snppopin2: none 

 of our progenitors to have intermarried with relatives, would 

 have had, at or about the period of the Norman conquest, up- 

 wards of a hundred million direct ancestors of that generation ; 

 and if we add the intermediate ancestors, double tiiat number. 

 As each individual has a male and female parent, we have oidy 

 to nudtiply by two for eaeli thirty years, the average duration of 

 a generation, and it will give the above rcsdlt. Let any one 

 assume that one of his ancestors at the time of the Norman eon- 

 quest was a Moor, anotiier a Celt, and a third a Ijaplander, and 

 that these three were preserved wiiile all the others were lost, lie 

 would never recognize either of them as his ancestor ; he would 

 only have the one hundred millionth i)art of the bloofl of each of 

 them, and, as far as they were concerned, there would be no per- 

 ceptible sign of identity of race. But the problem is more com- 

 plex than tluit which I have stated. At the time of the conquest 

 there were hardly a hundred million peojde in Europe. It follows 

 that a great mmiljer of the ancestors of the propositus must have 

 intermarried with relations ; and tiien the pedigree, going back to 

 the time of the conquest, instead of being n^presented by diverg- 

 ing lines. Would form a net-work so tangled tliat no skill could 

 unravt'l it. xlie law of probabilities would indicate that any two 

 peojile in the same country, taken at hazard, would not have many 

 generations to go back Ijcfore they would find a common ances- 

 tor, who probably, could they have seen him or her in the life, 

 had no traceable resemblance to either of them. Thus two ani- 

 mals of a very different form, and of what would be termed very 

 different species, might have a common geological ancestor, and 

 yet the skill of no comparative anatomist could trace the descent. 

 From the long-continued conventional habit of tracing pedi":rees 

 through the male ancestor, Ave forget, in talking of jn-ogenitors, 

 that each individual has a mother as well as a father, and there is 

 no reason to suppose that he has in him less of the blood of the 

 one than of the other. The recent discoveries in palieontology 

 show us that man existed on this planet at an epoch far anterior 

 to that commonly assigned to him." 



ORIGIX OF SPECIES IN INSECTS 



The following are the conclusions of Dr. D. B. Walsh, in a 

 paper on " Phytophagic Varieties and Species of Insects." This 

 name is given to those otherwise identical insects which differ, as 

 varieties or species, according to the species of plant they feed 

 upon. Diffei-ence of food, even when the food-plant belongs to 

 widely distinct botanical families, is accomj^anied by no difference 

 whatever, either in the larva, pupa, or perfect state, in many species 

 of insects. On the other hand, difference of food is in others ac- 

 companied by a marked difference in the color of silk-producing 

 secretions, in the colors, markings, size, structural differences, 

 and chemical properties of gall-producing secretions, in one or 

 both sexes, and in all stages of growth. From th(^ long catalogue 

 of facts enumerated by the author, he says: "For my own part. 



