In tracing the transition from lower to higher forms of life, we find a great change in the nature of the blood, or what answers to the blood, and the constitution of the blood is some index of the intensity of the metabolic processes going on within the organism.  The sap of plants is thin and watery, corresponding with the preponderant anabolism of the plant.  “Blood is a peculiar kind of sap,” and there is almost as much difference between this sap in warm-blooded and cold-blooded animals as between the latter and plants.  Rich, red blood characterizes the forms of life fitted for activity and bursts of energy.  In his exhaustive work on the blood Hayem has given a summary of the results of the investigations of chemists and physiologists on the differences in the composition of the blood in the two sexes.  Contrary to the assertion of Robin, Hayem finds that the white blood-corpuscles are not more numerous in women than in men, and he also states that the number of haematoblasts is the same in the two sexes.  All chemists are agreed, however, that the number of red corpuscles is greater in men than in women.  Nasse found in man 0.05824 of iron to 100, and in woman only 0.0499.  Becquerel and Rodier give 0.0565 for man, 0.0511 for woman, and Schmidt, Scherer, and others give similar results.  Welcker (using a chromometer) found between the corpuscles of man and woman the relation of 5 to 4.7, and Hayem confirmed this by numeration.  Cadet found in woman on the average 4.9 million corpuscles per cubic millimeter, and in man 5.2 million.  More recently Korniloff, using still another method—­the spectroscope of Vierordt—­has reached about the same result.  The proportion of red blood-corpuscles varies according to individual constitution, race, and sex.  In robust men Lacanu found 136 red corpuscles in 1,000; in weak men, only 116 in 1,000; in robust women, only 126 in 1,000; and in weak women, 117.[61] Professor Jones has taken the specific gravity of the blood of above 1,500 individuals of all ages and of both sexes.[62] An examination of his charts shows that the specific gravity of the male is higher than that of the female between the ages of 16 and 68.  Between the ages of 16 and 45 the average specific gravity of the male is about 1,058, and that of the female about 1,054.5.  At 45 years the specific gravity of the male begins to fall rapidly and that of the female to rise rapidly, and at 55 they are almost equal; but the male remains slightly higher until 68 years, when it falls below that of the female.  The period of marked difference in the specific gravity of the blood is thus seen to be coincident with the period of menstruation in the female.  A chart constructed by Leichtenstern, based upon observations on 191 individuals and showing variations in the amount of haemoglobin with age, is also reproduced by Professor Jones, suggesting that the variations in specific gravity of the blood with age and sex are closely related to variations in the amount of haemoglobin.