But, just at this point, we come upon the most striking results of recent research. Though all pollen is of this primary nature, named protoplasm, it is not found to hold true that pollen is of the same value for fertilization from whatever quarter it comes. On the contrary, most important differences result according to the source of the pollen. There is first the process of self-fertilization. But in many cases,--Mr. Darwin has shown that this holds of the majority of the orchids,--transference of the pollen from one plant to another proves to be a great advantage, if not an actual necessity for propagation of the plant. This process, known as cross-fertilization, gives a healthy and vigorous growth; want of it, will lead to degeneration, and ultimate extinction. This discovery has introduced a whole series of the most striking observations, throwing a flood of light on the distribution and interdependence of distinct forms of organism. The necessary relation between the pollen and the seed having been acknowledged, and next the value of transference of pollen from one plant to another, the first step in the line of discovery was made by the observation of a natural provision to _prevent self-fertilization_ by rendering it impossible that the pollen of a plant should fall on the seed of that plant. This entrance on the line of discovery was made by Sprengel so far back as 1790, by whom it was observed that in many plants the pollen and the seed did not come to ripeness at the same time.[BS] In some cases, the pollen is ripened before the seeds are ready; in other cases, the order of events is reversed. This observation naturally suggested transference of pollen from one plant to another; and this, connected with the continual coming and going of flies, bees, and butterflies, led to the further discovery, that _insects_ unwittingly perform a large part of the work needful in order to _provide for fertilization_. Mr. Darwin has pointed out that from the paper of Robert Brown in the _Linnean Transactions_, in 1833; and from that of Dr. Hooker in the _Philosophic Transactions_ for 1854, the peculiar phenomena had begun to awaken scientific interest. It was, however, when the researches as to origin of species had given fresh motive to observation concerning the relations of different types of organism, that the whole facts were brought to view, separately recorded, and at length systematized so as to lead to their full interpretation. Mr. Darwin himself concentrated on the orchids as peculiarly interesting and suggestive, while a host of workers all over the world were turning their energies into this new field of observation which promised ample return for patient research.