The gradualism of vitalism does not disappear with Darwinian evolution.
The gradualism of Lamarckian vitalism must still be evident in Darwinian evolution, just replaced by random and undirected trial-and-error…mutation/selection being the motive fuel instead of undefined, inner vital forces displaying outward purpose and direction.
The gradual continuity in biology does not go away with Darwinism. In Darwinism it merely shifts over into the worldview of naturalistic materialism.
But surprisingly, the concept that species are immutable becomes a non-issue when either a continuous chain of increasing complexity is produced one new and different cell-type at a time (nature makes no sudden leaps), or new cell-types are introduced in grouped clusters producing instantly mature function (nature making recognizable leaps as seen in the fossil record).
Continuity exists in both cases.
But the one new-and-different cell-type at-a-time program must proceed within the materialism of small-step transitions achievable through trial-and-error natural selection, whereas the other program of grouped clusters of new cell-types must rely upon an outside intelligent agent releasing the packages of information that can create new amino acid folds to produce new proteins…to then construct new and different cell-types to build elephants rather than giraffes.
Both vitalism and Darwinian evolution need more terrain for trial-and-error, progressive development, which we do not observe in nature.
The question of whether species are immutable or not is only an issue of concern within a materialistic worldview that by definition cannot and will not admit intelligent agency.
In materialism, continuity one small-step at a time (nature makes no sudden leaps) is the glass ceiling that limits biology.
But modern science has added evidentiary meat to the bones of the ancient Greek debate regarding a material or non-material causation of this universe.
The evidentiary facts do not identify who the intelligent designing agent is, but they factually make the case for the inescapable need for intelligent agency underlying the organized complexity we see everywhere in the natural world.
Where the ancient Greek philosophers argued pre-scientifically through logic using speculation in-place of empirical evidence, modern science has replaced speculative conjecture with complex, specified, and coherently integrated systems of information to describe much of the causations of phenomena in the natural world.
Species are easily mutable if the God of the Bible is releasing through gene regulatory networks and epigenetic factors, entire blocks of new and different cell-types in clustered groups that produce immediate function, as the discontinuities in the geological record clearly show.
The fantastic irony in 2022 is that the “hopeful monster” hypothesis of evolution by saltation put forward by Goldschmidt in his 1940 book The Material Basis of Evolution, in an attempt to close the obvious gaps between living organisms in the common descent program, is that he is correct if we replace within the equation of biological development, the factor of scientific materialism with the alternate factor of intelligent agency.
The infinitude of connecting links required by materialism disappears when the simple move from one new and different cell-type at a time, is made to adopting the more plausible program of new and different cell-types being introduced in blocks of clustered groups.
This better concept produces new and varied life-forms entering into existence at the end-point of functional development to immediately engage into the fast-lane of survival and reproduction.
Intelligent agency working within width-less durations of zero time to produce immediate function at the end-points of development, removes the untenable requirement of a vast larger sized earth having more surface area, to provide the increased terrain for trial-and-error, accidental development to play-out.
Intelligent agency places the creative process genetically inside living cells where we know there is adequate DNA information to produce the vast diversity of life, rather than the externally-driven impetus of mutation/selection through survival pressures that produces only the microevolutionary changes we see in Darwin’s finches on the Galapagos islands.
 A worldview prior to Darwin that attempted to explain the design apparent in nature to as-yet not understood inner driving natural forces, termed as vitalism.