The truth is Darwin is junk science, it fails Darwin’s own tests due to lack of evidence. By his own words Darwin would have abandoned his own theory long ago.
You are probably referring to Darwin's statement that the lack of transitional fossils posed a serious problem for his theory, and that if they weren't found in the future his theory would be in trouble.
Guess what? Darwin was speaking of the fossil record as it existed prior to 1859.
There are lots of transitionals now, and more are found every year. There was just an announcement in the last week about a new whale transitional. And if you want to see living transitionals, just google "ring species" for examples of speciation with the transitional populations still intact and alive.
The problem seems to be that some folks with particular religious beliefs are unable to admit there are transitionals because that would contradict their beliefs. But denial won't make all of the transitionals magically go away.
Three Words:
Piltdown Man Hoax.
Still defended by the most ardent Evolutionist Zealots..
You don’t even need to go to something as exotic as ring species. (Even though creatures such as Darwin finches aren’t exactly “exotic”, most people aren’t particularly familiar with them.)
Even two species as familiar as the lion and the tiger comprise a snapshot of evolution in action. Lions and tigers are almost-but-not-quite reproductively isolated from one another (the definition of a species). That is, they do not normally mate, and when they do, the offspring is usually not viable; when it is viable, it is sterile. Therefore, any mutative adaptations among lions cannot transfer to the tiger population and vice versa. They are genetic islands, i.e. species.
There is one exception to this, however. If a female lion and a male tiger mate, their offspring will be an animal called a tigon. Male tigons are sterile, but female tigons can mate with either lions or tigers (producing animals called ti-tigons or li-tigons, which are then fairly representative of just a tiger or just a lion). Through this extremely narrow hybridization channel, lions and tigers can still interbreed.
What would it take for full speciation to occur between tigers and lions? Well, presumably there’s some gene on the male tiger’s Y chromosome that makes gamete manufacture malfunction in the context of some set of chromosomes from a lion - thereby rendering male tigons (with their tiger father’s Y chromosome and their lion mother’s X chromosome) sterile, but leaving female tigons (with one X chromosome apiece from their father and mother) fertile. It would be possible, due to mutation or transchromosomal crossover, for that gamete-breaking gene on the Y chromosome of some male tiger somewhere to move over to that tiger’s X chromosome (presumably this would occur during fertilization when that male tiger gets conceived). All of that tiger’s daughters would have a copy of that modified X chromosome - and as they have offspring in turn, that gene would spread throughout the tiger population until the ability for tigers to form fertile offspring with lions in any way whatsoever would be a vanishingly unlikely phenomenon.
And I am sure it will stand the test of time just as Nebraska man, Piltdown man, and the Archaeoptrix finds did (they didn’t). People will write papers for dozens of years and stake their whole scientific careers on it, only to down the road be proven to have based their whole careers on a fraud.
Just chalk coyoteman up there with Nebraska man as far as I’m concerned.
Then why couldn't the late, great Stephen Gould see any of them? He spoke frequently of the complete absence of them. Was he a crypto-creationist?