It is not always the magnitude of the differences observed between species that must determine specific distinctions, but the constant preservation of those differences in reproduction.
Interpretation
The importance of how differences between species are maintained through reproduction is more significant than their size.
Jean-Baptiste Lamarck emphasizes that while observable differences between species can vary greatly, it is the consistency in how these differences are passed on through reproduction that truly defines the uniqueness of each species. This highlights the role of heredity and reproduction in evolutionary biology, suggesting that the stability of traits is fundamental to the identification of species rather than just the extent of their differences.
In practice
In a biology class discussing evolution, this quote could be used to explain how traits are inherited.
Now this circumscribed power, which we have scarcely examined, scarcely studied, this power to whose actions we nearly always attribute an intention and a goal, this power, finally, that always does necessarily the same things in the same circumstances and nevertheless does so many and such admirable ones, is what we call 'nature' .
All knowledge that is not the real product of observation, or of consequences deduced from observation, is entirely groundless and illusory.
It is not enough to discover and prove a useful truth previously unknown, but that it is necessary also to be able to propagate it and get it recognized.
Now, a living organism is nothing but a wonderful machine endowed with the most marvellous properties and set going by means of the most complex and delicate mechanism.
Lest we forget, the birth of modern physics and cosmology was achieved by Galileo, Kepler and Newton breaking free not from the close confining prison of faith (all three were believing Christians, of one sort or another) but from the enormous burden of the millennial authority of Aristotelian science. The scientific revolution of the sixteenth and seventeenth centuries was not a revival of Hellenistic science but its final defeat.
The typical imperative from biology is not "Thou shalt... ," but "If ... then ... else.
We live in a dancing matrix of viruses; they dart, rather like bees, from organism to organism, from plant to insect to mammal to me and back again, and into the sea, tugging along pieces of this genome, strings of genes from that, transplanting grafts of DNA, passing around heredity as though at a great party.
Mechanics is the paradise of the mathematical sciences because by means of it one comes to the fruits of mathematics.
Wherever we look at the living biota … discontinuities are overwhelmingly frequent…The discontinuities are even more striking in the fossil record. New species usually appear in the fossil record suddenly, not connected with their ancestors by a series of intermediates.
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