Originally Darwin posited a monophyletic beginning to life if you like, where on the phylogenetic, evolutionary tree, the trunk lead down to the bottom where there was one original common ancestor.
Creationists believe the opposite, that when God created life on earth it began as a polyphyletic beginning, so to speak, with many phyla existing from the beginning of time.
With that in mind, if created kinds exist as opposed to Darwins evolution of phyla, we expect the fossil record if it is to fit with created kinds or baramins, to show evidence of abrupt appearance of fully formed phyla with no trunk, with no intermediates or connections showing how it evolved.
We see a lawn, the true evidence doesn't fit with Darwinism but in fact shows abrupt appearance and stasis. This is what we would expect with created kinds.
So now as to the issue of DEFINING KINDS. What I wanted to show firstly in this thread is that there is strong confirmation that phyla appear in the record and their species will remain unchanged, which means that genetically speaking, it is factual that life is reproducing what it is. (Law Of Identity).
That is to say, even if creationists really did struggle to define kind, it would not follow that they therefore don't exist because animals are still reproducing the same body plans, certain types of animal can be shown to have reproduced the same types of animal, and even if you accept long ages, starfish or jellyfish in the Cambrian are identical to todays apart from size, the same for millipedes at 400 myo, and dragon-flies.
So the first point is, if we do genuinely struggle to define kinds, if they did originally exist, gene pools are still reproducing the same things. We know that genetically, we can certainly recognise types of animals which don't change even long-term. We know there is a limit generally but to then say we should know a limit specifically, would be to commit a sweeping-generalisation.
In other words, we can't define kinds as a classification, because the limits for specific cases, and all the issues surrounding those limits or even non-limits, make it impossible. That is to say, for one type of animal it's kind may be at the level of genera but for another type it could be at the level of family.
So then to define kind the best way of defining it, is that it is the original gene pools from a polyphyletic beginning to life, from the existing species that have descended from them. Or a better way of saying this might be that todays species reproduce according to their genetic identity. We know a human won't give birth to a chicken and that humans will only reproduce things within the homo genus. (for humans, there is a human kind and we pretty much know it is at the genus-level, with various species within it such as Neanderthal, Erectus, Sapien, etc..) The only reason we know the human is a kind is because the bible tells us that was the original kind, a man and woman. It doesn't say this for other creatures as it doesn't give their numbers and types. Logically we only know of one kind, technically speaking, and we only have that knowledge because the bible tells us the kind started with a man and woman.
THE STRUGGLE TO CLASSIFY KINDS
Evolutionists take advantage of creationists struggling to place kinds, but this isn't the same as defining them. For the definition, we only have to argue what we see in nature, that animals reproduce according to what they are. Certainly this is the case even if we can't know what they originally are.
The problem isn't that kinds don't exist. The problem is we don't know the original traits in the original gene pools because what they would consist of would be both extant and extinct species. I hope you can see the problem, if there used to be a cat with horns, but we don't know of it's existence as it has not been preserved in the rock record, then how can we say, "all cats consist of three kinds, and here is what they are". That's impossible because, 1. We don't have knowledge of all of the species and don't know which ones have gone extinct if any. 2. If there was genetic information for horns in a cat species, how can we define three kinds and not include that potential anatomy?
There are also two very genuine problem with kinds, and that is some organisms appear identical in nature but are not the same kind of creature but have totally different, unrelated genetics. That is one problem. Another problem is that we can find amazing similarities in nature and we know this doesn't mean there is necessarily a relation even though it may seem like it's possible. For example even evolutionists accept that tortoises and turtles are not at all related, however the fact remains they certainly look anatomically similar to the point where it might be tempting to say they are of the same kind. This is a genuine problem because it may seem for example that we can say, "there is a bat kind" but in fact there is a very strong possibility that God made creatures like cats or bats, but that he made them into separate kinds like He did with turtles and tortoises. It seems very clear that there is no evolutionary reason why turtles should look so similar to tortoises, so the obvious reason they exist is that God wanted a water version as well as a land version. In that regard, it is almost impossible to know for example, if bats or cats or dogs are a kind, or whether some of them are their own kind.
By analogy imagine if the world became post-apocalyptic and all names on the remaining cars, all logos, badges, were not present any more, but cars were shells so to speak. If we could not define which cars were of the Ferrari kind, any more but we could perhaps guess that 60% of them were probably of the Ferrari type, would it logically follow that if we couldn't properly define the kinds/types of car, that originally they therefore did not exist? No, because if they started to reproduce the same cars, they would still be reproducing according to their identity, by replication. (analogous to reproduction).
In other words, Ferraris would still only lead to Ferraris even if we did not know they were Ferraris, if reproduction of those types of cars continued. Proving our confusion about kinds, is irrelevant, logically.
So then animal kinds still reproduce animal kinds of the same type according to gene pools, even if we don't know the defining parameters for specific cases, for this won't change reality being reality, our level of knowledge won't change history, so it is a non-sequitur to conclude that because creationists might struggle to nail down specific kinds that therefore they don't exist. Certainly we see in breeding that there are limits to what we can get, with dogs, pigeons, etc..so we know an obedience-to-type is occurring in nature, which I believe is all we need because we aren't seeing any evidence of macro evolution, but what we tend to see in nature, is something referred to as reversion to the mean. (You can hear about this in the following video between 7.50 and 9 minutes; breeders already know this is factual;