268 GENERAL BIOLOGY 



that they look alike. The robins on the lawn may be 

 known personally by any one who will take the trouble to 

 note personal characteristics. 



Nature abounds in little refinements of structure, such as 

 we see in the raised lines traversing the cuticle of our finger 

 tips. These lines are never exactly alike in any two per- 

 sons. So distinct are these differences that finger prints are 

 now-a-days a well recognized aid to the identification of 

 criminals. No two leaves on any tree are exactly alike; 

 indeed those on the same tree may exhibit differences that 

 are very marked (fig. 162). 



Fluctuating variations. The differences between the 

 individuals of a species extend to every personal character- 

 istic: stature, strength, activity, temperament, etc., but they 



are usually slight, and fluct- 

 FIG. 163. A six-spined seed uate about a mean that 



of the rag weed. . n - 



expresses the normal con- 

 dition for the species. This may be simply illus- 

 trated by the seeds of the common rag weed (fig. 

 163) each of which bears a long apical point, sur- 

 rounded by a circle of short spines. The normal 

 number of these spines appears to be six, but 

 many seeds have five or seven of these spines, and a few have 

 even smaller or greater numbers of them. A count of the 

 spines on 100 seeds taken at random gives the following 

 results : 



No. of spines 1234 5 6 7 89 



No. of times occurring i 3 7 9 25 37 25 12 i 

 If now the seeds of each class be arranged in columns, and 

 aline be drawn joining the tops of the columns, that line will 

 be the curve of variation (fig. 164), a common iheans of 

 expressing variations of this type. 



The class containing the greatest number of seeds (called 

 the mode; the six spined class in this case) may be regarded 



